From 1d4chan

The Expert was one of several NPC Classes introduced in Dungeons & Dragons 3rd edition. An Expert is skilled in a profession or craft. The class has its roots in the old, OLD school D&D as the retainer with some specialised profession, from alchemist to zookeeper. With "porter" in between.

Essentially, they're meant to represent anyone who has a well-trained job past "nondescript civilian" (so not a commoner), but also doesn't fit into any traditional PC class. This is expressed by the fact that they pick any ten skills at character creation to be their class skills, as well as getting an above-average six skill points to work with. So, for instance, a blacksmith could be an expert who chose mostly Craft-related skills, a scribe, scholar, or librarian might pick up various Knowledges and Decipher Script, a diplomat would select as many social skills as possible, and so on.

Notably, since they can pick any ten skills, they can actually be min-maxed somewhat-effectively compared to their non-spellcasting NPC class brethren. In particular, Iaijutsu Focus can give them a solid damage option, Use Magic Device turns them into a knockoff caster, Tumble grants excellent mobility, Autohypnosis can make them immune to fear and poison, the social trifecta of Bluff, Diplomacy, and Intimidate lets them handle all face duties, and that still leaves two slots for things like Sense Motive, Spot/Listen, Handle Animal, Stealth, or Survival. There are also some silly builds that revolve around skill checks rather than other mechanics.

Perhaps because of this, the class was, ironically, power-creeped by the Factotum and Savant classes, who automatically get all skills as class skills without having to pick, and get actual class features. On the other side this class meant... who even needed to play a Rogue anymore.

The Expert returns in 5th edition as a sidekick class. The expert gets more proficiencies than the other sidekick classes and its class features for combat center around using the help action to assist teammates.

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
Dungeons & Dragons 5th Edition Classes
Barbarian - Bard - Cleric - Druid - Fighter - Monk
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
Tasha's Cauldron of Everything: Artificer - Expert - Spellcaster - Warrior
Van Richten's Guide to Ravenloft: Apprentice - Disciple - Sneak - Squire
Unearthed Arcana: Mystic