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.

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.

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.

In Warhammer 40,000

For Eldar Warlock, see here.