From 1d4chan

The Adept was one of several NPC Classes introduced in Dungeons & Dragons 3rd edition, a direct response to NPCs and PCs now running off of the same system. It takes the place of Wokani from BECMI, sharing many of the same spells and maximum of 6th level. The other NPC Classes of that edition were the Aristocrat, the Commoner, the Expert and the Warrior, plus Eberron's later addition of the Magewright.

The Adept was a divine spellcasting class that mingled elements of cleric and wizard. It could cast divine spells of up to 5th level and prepared them in advance, but it also possessed a familiar. It's very slightly less squishy than a Wizard or Sorcerer and as a divine caster can wear armor (though it isn't proficient in any). Eberron gave the Adept class a major buff by giving them one domain, gaining the spells (including, by RAW, those of 7th, 8th and 9th level, enabling activation of magic items with them and, with effort, even cheesier stuff) and powers of that domain.

Infamously Adept is tier 4 thanks to the ability to handle a handful of encounters really well while struggling at others due to the limited spell list. This is above the other NPC classes, and actually higher tier than the monk, the paladin, and the fighter. Yes, really. This was a spellcaster's edition, son! Unlike other core caster classes however, only four (all from Complete Champion) new Adept spells were added in the entirety of the system's life. Masters of the Wild suggested a variety of spells that should be added to the adept list, however, and Invoke the Cerulean Sign (Lords of Madness) explicitly says it can be mastered by any spellcasting class. It did however, oddly enough, get a prestige class virtually exclusive to it (it requires Lightning Bolt as a divine spell, something only it, Archivist and Shugenja can do and those two need to get survival as a class skill somehow) in the Hexer.

Pathfinder's only change to adept was the system wide skill changes. Pathfinder's adept has slightly more non-core spells, though none of them are great. It also has a single feat, Adept Channel, with requirements that ensure pretty much only Adepts can take.

Dungeons & Dragons 3rd Edition Classes
Player's Handbook: Barbarian - Bard - Cleric - Druid - Fighter - Monk
Paladin - Ranger - Rogue - Sorcerer - Wizard
Player's Handbook II: Beguiler - Dragon Shaman - Duskblade - Knight
Complete Adventurer: Exemplar - Ninja - Scout - Spellthief
Complete Arcane: Warlock - Warmage - Wu jen
Complete Divine: Favored Soul - Shugenja - Spirit Shaman
Complete Psionic: Ardent - Divine Mind - Erudite - Lurk
Complete Warrior: Hexblade - Samurai - Swashbuckler
Dragon Compendium: Battle Dancer - Death Master - Jester
Mounteback - Savant - Sha'ir - Urban Druid
Dragon Magazine: Sha'ir - Deathwalker - Fleshcrafter - Soul Reaper
Dragon Magic: Dragonfire Adept
Dungeonscape: Factotum
Eberron Campaign Setting: Artificer
Heroes of Horror: Archivist - Dread Necromancer
Magic of Incarnum: Incarnate - Soulborn - Totemist
Miniatures Handbook: Favored Soul - Healer - Marshal - Warmage
Ghostwalk: Eidolon (Eidoloncer)
Oriental Adventures: Samurai - Shaman - Shugenja - Sohei - Wu jen
Psionics Handbook: Psion - Psychic Warrior - Soulknife - Wilder
Tome of Battle: Crusader - Swordsage - Warblade
Tome of Magic: Binder - Shadowcaster - Truenamer
War of the Lance: Master
NPC Classes: Adept - Aristocrat - Commoner - Expert - Magewright - Warrior
Second Party: Mariner - Mystic - Noble - Prophet
Class-related things: Favored Class - Gestalt character - Multiclassing
Prestige Class - Variant Classes - Epic Levels - Racial Paragon Classes