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 PHENOMENAL COSMIC POWAH 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 other words: a wizard gets "A"s in class by studying really hard, a sorcerer gets "A"s by having natural talent, and a warlock gets "A"s by giving the dean a blowjob in his office.

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. It's basically the 3rd Ed Binder, making contracts with Primordials, dead gods, and ancient warriors, and win out in sheer variety regarding damage types, powers, and such. the Forgotten Realms handbook introduced the Dark Pact Warlock, who have made deals with things in the Underdark (poison, necrotic, no dual scimitars). "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. 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.

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.

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

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.