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 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.

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.

The 3.5 ed Vestige Pact Warlock was introduced in Complete Arcane. It looks like textbook munchkin bait, but actually sucks. Warlocks have at-will casting and no spells per day in 3.5, which made some people call hax but isn't really that bad 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. It is generally recognized that at the end of the day, one of the Warlock's most powerful abilities is to emulate DAKKADAKKADAKKA with Eldritch Blasts, without having an accuracy problem. This ability alone can brake the game, but the time necessary to do so balances it out, so this class is both for people who want simple, fast combat while going The Matrix on the ceiling and for patient masterminds.

Pathfinder didn't give the warlock a conversion. Oh, sure, the witch is now a class, and it has some vague similarities of flavor, but they're mechanically too different to compare.

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).

The 5e Warlock is not only balls to the wall awesome, but it's also possibly the best marriage of 3.5 and 4e design imaginable. Warlocks get Vancian 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 basically the sickly pale white kids in gym class that had asthma attacks about halfway through the first lap around the track, but in reality they're the roid-pumping nightmares that you see in afterschool specials; Their spell slots all automatically scale the slot level up so their spells are always guaranteed to be pumped up, and their spell slots also refresh after every encounter. Invocations are back from 3.5, and they're of the similar concept, 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, to power-ups for Eldritch Blast. As icing on the cake of the Warlock package, All Warlocks can get the benefits of a pact with a supernatural being. At this point the only pact options are the Sexy Satyr, the Sinful Succubus, and Shub-Niggurath. The pact you choose gives you extra spells of a related theme (for example, Archefey giving Faerie Fire, Fiend giving Fireball and Great Old One giving Evard's Black Tentacles) and a bevvy of unique class features like teleportation, damage resistance and 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 suped-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.

4e and 5e 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.

In Warhammer 40,000

A fan's interpretation on how an Altansar Warlock should look like. No wonder, since one very well known Phoenix Lord that comes from Altansar has a lot of skulls incorporated into his armour.

Warlocks are Eldar psykers. Unlike Farseers, who specialize in telling the future and typically follow the path of the Seer for their entire lives, warlocks were at one point on the Path of the Warrior. Due to this mixing of psychic potential and warrior background, Warlocks specialize in burning shit down. With their minds.

They are often seen accompanying Eldar Guardians. Some specialize in leading Wraithkind, and these are called Spiritseers and are typically found accompanying Wraithguard around. They can also make up a Seer Council, which is basically a 10-elf squad of warlocks that serve as a Farseer's bodyguard.