Warlock

Warlocks are a type of magic spellcaster in fantasy settings. They are also the male counterpart of witches.

Compared to others, warlocks are usually dedicated completely to offensive magic and in a few cases, dabbling in the forbidden arts like black magic and daemonology to achieve more power, although this is not always the case.

In Deadlands

In Deadlands, both the generic black magic-using badguy and the PC "huckster" class/archetype/thingy would technically fall under the warlock moniker, and black magicians are commonly called warlocks to boot. Hucksters are a special case in that they literally deal with the devil for their magic; whenever a huckster casts a spell, they astrally project into the Spirit World and offer a passing demon to play a game of skill & chance (poker being, of course, traditional); if the demon wins, it gets a chunk of the huckster's soul (which translates to a huge amount of physical pain and/or death for the huckster), and if the huckster wins, the demon has to surrender the mojo needed to make the huckster's spell a reality. Part of what makes spells differ in difficulty to cast is the some spells either need more juice to work than others or else are calling for stuff the demon wouldn't normally like to do - it's a lot easier to call them up to kill somebitch than to make them protect you, for example.

In Dungeons and Dragons

A Warlock class character.

Instead of gaining their power though meticulous training and study like the Wizard, or natural-born talent like the Sorcerer, they make contracts with very powerful entities and forces and channel that energy. Like an Arcane cleric almost.

1e

In 1st ed AD&D, characters had titles as they leveled up. An 8th level Magic-User gained the title "Warlock." Nobody cared about these titles, not even the people who used weapon speed or the AC to-hit modifiers for each weapon. There was a Dragon magazine article (issue 43) describing a 'witch' npc class, and male npcs that took this class were sometimes called "warlocks" instead.

2e

In 2nd ed AD&D, a character could take a class kit, which was essentially a set of alternate class features that you could take to add a different play style to a class. Such kits required you to meet certain requirements to take them though. In the The Complete Wizard's Handbook, there was a Wizard kit called the Witch. In the description of the Witch kit, it is mentioned that most Witches are female, but male Witches are possible, being commonly referred to as Warlocks. The Witch kit states that the power the Witch gains, is taught to them by extraplanar entities, for a variety of reasons. In exchange for this magical knowledge though, Witches constantly had to struggle with the extraplanar entities to maintain their free will. Unlike traditional Wizards, a Witch needed to be more than just Intelligent, requiring decent Wisdom and Constitution scores as well. While this kit is not an official Warlock class, it was the first time the themes of the Warlock made it into a player class.

3.5

The 3.5 ed Warlock was introduced in Complete Arcane. It looks like textbook munchkin bait, but is actually kinda meh. Warlocks have at-will casting and no spells per day in 3.5, which made some people call hax but isn't so hot because very few DMs would ever run enough encounters in one in-game day for Vancian casters to completely run out of spells. Among the ones the handful that do, some like to have spells that are at-will and keep 3.5. One of the Warlock's most powerful abilities is to DAKKADAKKADAKKA with Eldritch Blasts, without having an accuracy problem. This ability can win encounters, but the time necessary to do so balances it out, so this class is both for people who want fast combat going The Matrix on the ceiling, and for patient masterminds.

Pathfinder

Pathfinder didn't give the warlock a conversion due to their non OGL status. Occult Adventures, however, introduced the Kineticist class, which follows Warlock mechanically a bit more closely than Magus to Duskblade, but thematically you are an element bender instead of having made vague deals with evil outsiders or fey. The witch takes up the flavor portion of the warlock while having its own mechanics.

Then Ultimate intrigue came along and made warlock an archetype of the Vigilante. In this context, they are more Vigilante Arcane spellcasters. While they have some magus spellcasting, they also can sling magical bolts, and still use almost all the vigilante tricks of misdirection, masked combat, and walking straight past the guards after making a quick change of outfits.

The Classes of Pathfinder 1st Edition
Core Classes:	Barbarian - Bard - Cleric - Druid - Fighter - Monk Paladin - Ranger - Rogue - Sorcerer - Wizard
Advanced Player's Guide:	Alchemist - Antipaladin - Cavalier Inquisitor - Oracle - Summoner - Witch
Advanced Class Guide:	Arcanist - Bloodrager - Brawler - Hunter - Investigator Shaman - Skald - Slayer - Swashbuckler - Warpriest
Occult Adventures:	Kineticist - Medium - Mesmerist Occultist - Psychic - Spiritualist
Ultimate X:	Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante

4e

In 4e, Warlocks are divided into pacts. In the first player's handbook, we got the Infernal, Fey, and Star Pact Warlocks. Infernal Warlocks channel demonic energy, and need a high CON score to avoid succumbing to waves of demonic power. They also want an above-average INT, to make the most of their powers. Fey Warlocks made Pacts with Fae, and most of their powers involve Charm effects and teleportation. They use a high CHA score. Star Pact warlocks basically made a deal with the C'Tan or Cthulhu, and they manipulate fate, rolls, and what have you. However, they need to focus on three stat scores, leading to heavy MAD. The plus side of playing a Star Pact Warlock is you get all the coolest powers. The Forgotten Realms handbook introduced the Dark Pact Warlock, who have made deals with things in the Underdark (poison, necrotic, no dual scimitars). The Arcane Power sourcebook gave us the Vestige pact, which is basically the 3rd Ed Binder, making contracts with Primordials, dead gods, and ancient warriors, and wins out in sheer variety regarding damage types, powers, and such. "Heroes of Shadow" has given us two new ones, one for Essentials and one for Core, that are heavily implied to take their powers from the Darklords and/or Dark Powers of Ravenloft, which kind of makes up for the whole Vampire thing.

So in short, Warlocks make pacts with the Powers that Be; namely Before (Vestige Pact), Below (Infernal Pact), Beneath (Dark Pact), Behind (Fey Pact) and Beyond (Star Pact).

4e paired the Warlock class up with the Tiefling race, much like how Dwarves make iconic Fighters or Clerics, or Elves make good Wizards, or Half-orcs make good Barbarians. 5e continues this tradition.

5e

The 5e Warlock is a complicated marriage of 3e invocations, 4e pacts, and 5e spellcasting, with a few caveats. They gain a pitiful amount of spell slots, as well as only a handful of spells known. To the untrained player's eye, they're painfully limited compared to "real" casters, but in reality they're roid-pumping nightmares not bad. Their spell slots all automatically scale the slot level up so their spells are always guaranteed to be pumped up, and also refresh after every encounter or few (short rest). Invocations are back from 3.5, albeit scaled back a bit, providing various kinds of special abilities, from access to spells that aren't on their list as at-will, encounter or daily powers, to power-ups for the pact form, and of course power-ups for Eldritch Blast.

To complete the Warlock package, you get a pact with a supernatural being. The pact gives you a thematic extended spell list; for example, Great Old One Warlocks can cast (but not spam, sadly) Evard's Black Tentacles. Pacts provide a bevvy of unique class features like teleportation, damage resistance or telepathy. On top of magical powers, they also give you a consolation prize in the form of a pact boon, which can either be a souped-up familiar, a free enchanted weapon that you can shapeshift into whatever kind of weapon you need, or a magical book that gives you three cantrips from any other spellcasting class. An alternative pact boon was the Star Chain, introduced in "Non Divine Faithful", which closely ties into the Seeker patron: this is a magical trinket that can be used to cast Augury and can be used to grant Advantage on an Int check once per short rest.

The available pacts are:

Archfey - Bound to a powerful faerie lord or sylvan pseudo-deity, a warlock with this pact gains the power of Fey Presence (Charm or Frighten all creatures in a 10ft cube around you once per short rest), Misty Escape (teleport 60 feet and turn invisible once per short rest), Beguiling Defenses (you are immune to Charming and can attempt to Charm anyone foolish enough to try a Charm effect on you) and Dark Delirium (can attempt to entrap a target in an illusion to Charm or Terrify it once per short rest). This one is in the Player's Handbook.

Fiend - Bound to a Demon Prince or Archdevil, a warlock with this pact gains Dark One's Blessing (gain temporary HP for dropping another creature to zero HP), Dark One's Luck (can choose to re-roll an ability check or saving throw with a +10 bonus once per short rest), Fiendish Resilience (gain Resistance to a single damage type of your choice after completing a short rest, though magic and silvered weapons can pierce it) and Hurl Through Hell (teleport a target into some hellish dimension for 1 turn, causing 10D10 Psychic damage to a non-fiendish target, once per long rest). This one is in the Player's Handbook.

Great Old One - Warlocks with this pact are bound to terrible abominations from outside time and space, drawing on the power of aberrant gods like Cthulhu. They gain the boons of Awakened Mind (telepathy with a 30ft range), Entropic Ward (can make yourself harder to hit and get a bonus if the target missed once per short rest), Thought Shield (mind cannot be involuntarily read, Resistance to Psychic Damage, inflict equal Psychic damage on anyone who inflicts Psychic damage on you) and Create Thrall (permanently charm a single creature with a touch, gaining telepathic communication with them from anywhere on the same plane). This one is in the Player's Handbook.

Undying - A Warlock with this pact has offered their soul to something that has "cheated death"; most obviously a powerful lich, ghost or vampire, but mortals ascended to godhood and weirder things are also valid. The Nameless One isn't mentioned, but would be a perfectly thematic (and awesome!) patron for this pact. The "necromantic" pact, these warlocks get features related to the ability to cheat death; Among the Dead (Spare the Dying as a bonus cantrip, Advantage on saving throws against disease, Undead must pass a Wisdom save to attack you), Defy Death (gain a significant healing 1/day by passing a death saving throw or using Spare the Dying), Undying Nature (can hold your breath indefinitely, don't need food, water or sleep, immunity to magical aging, age 1 year for every 10 you live) and Indestructible Life (can freely heal yourself 1/encounter, including reattaching limbs as part of it). Comes from the Sword Coast Adventurer's Guide.

Ghost in the Machine - A Warlock with this pact comes from a hyper-dungeonpunk, high-magitek or urban fantasy type setting, having made a pledge with some sort of powerful artificial intelligence. This warlock gains the ability to mentally control computers, teleport along electrical wires & data cables, resistance to magical forms of detection and information retrieval, and the ability to "infect" others with a techno-virus by touching them, which hurts them with Psychic damage and lets you control them like puppets. Comes from the "Unearthed Arcana: Modern Magic" article. This is awesome, but sadly useless in the standard high fantasy setting your group insisted on playing in.

Undying Light - Making a pledge to the force of Positive Energy itself, these warlocks are a weird melding of warlock and cleric, making them a pretty interesting alternate take on the Favored Soul. In addition to a bunch of fire/light based spells, including two new cantrips, it gives Radiant resistance, a bonus to Radiant or Fire damage that it inflicts, increased vitality, and a healing touch. It also comes with a d6 table of optional personality flaws, which basically revolve around hating the dark and loving the light - despising the undead, being afraid of the dark, always needing to light up any place that's dark, things like that. Comes from the "Unearthed Arcana: Light, Dark, Underdark" article.

Seeker - The Seeker is a semi-divine warlock, sworn to the service of a deity or similar entity dedicated to gathering lore and knowledge. The UA article it appears in outright states that it mostly owes its inspiration to Celestian, the Greyhawk god of stars, space and wanderers. At 1st level, it grants the Shielding Aurora feature, where you can invoke a 1-turn-long forcefield once per short rest that grants you Resistance (All) and inflicts Radiant damage on any enemy that ends their turn within 10 feet of you. At level 6, it grants Astral Refuse; spend an action and you teleport off the battlefield to a non-space, where you can cast two spells on yourself before returning to where you started and ending your turn. 10th level grants you the Far Wanderer feature; you no longer need to breathe and are now Resistant to Fire and Frost damage. Finally, at level 14, you gain the Astral Sequestion feature: once per day, you can spend five minutes performing a ritual that teleports you and your party (no more than 10 people plus yourself, mind) to the Astral Plane. There, you get to complete a short rest before returning to the material plane with no time having passed in the interim. Its bonus spells are a mixture of divination and mobility enhancers, from Feather Fall to Legend Lore.

Hexblade - Pledge yourself to a sentient weapon of some description. Echoes the old Hexblade class and focuses on melee combat (it's a pretty obvious pairing with the Pact of the Blade). You get spells focused on making your weapon more magical (including three Paladin smite spells!), boosting your mobility, and a few kill-y spells. You also get proficiency with medium armor, martial weapons, and shields, and can substitute your charisma for dex or strength when making attacks with 1handed weapons. Other features include a curse you can put on people to make it easier to kill them (grants increased crit threshold, add proficiency to damage against them, regain hitpoints when they die, eventually makes it hard for them to hit you). Finally, you get a magic dog that lives in your shadow. You can make this dog go into someone else's shadow, which will let you ignore most cover they might be behind (I know that sounds made up, but it's real, I swear.)

Raven Queen - Pledge yourself to the Raven queen. Focuses mostly around having a raven familiar. Spells are themed around death, finding things, and cold. Features give you a magic raven that grants you some bonuses, which you can eventually merge with. You also get advantage on death saving throws, immunity to frighten, and resistance to necrotic damage, and the ability to cast "Finger of Death" once per long rest.

Celestial - This is a refluffed version of the Undying Light Patron presented in the June 2017 UA and implied to be planned for the Xanathar's Guide to Everything. The big difference is that it adds some healing spells to its list of bonus spells, instead of just fire/radiant damage-dealers, and rearranges what levels you get which features at. At 1st level, you gain Healing Light - the "heal with a touch" feature from the UL Patron, which it didn't get until level 14 - and Light & Sacred Flame as bonus cantrips. At level it gets Radiant Soul, which is Resistance (Radiance) and bonus to radiant & fire damage, a feature the UL got at 1st level. They both get the same "bonus temporary HP on completing a rest" feature at level 10, but the Celestial renames it the Celestial Resilience trait. Finally, it gets Searing Vengeance - 1/day, when reduced to death, spring up at half maximum hitpoints and inflict radiant damage & blindness on all enemies within 30 feet - as its 14th level trait, when for the UL Patron it was a 6th level feature.

In Warhammer 40,000

For Eldar Warlock, see here.