There are four main types of
Dungeons & Dragons* professions:
fighting men,
magic-users,
clerics, and
thieves.
Fighting Men can use any kind of weapon, and wear any kind of armor. They cannot perform magic (though they can use magic armor and weapons).
Magic-Users cannot wear any armor, and may only use a dagger as a weapon. They can use any magic item other than armor or weapons (except magic daggers, of course). They can cast spells.
Clerics can wear any kind of armor, and use any non-edged weapons (such as maces and flails), including magical versions. They are dedicated to the god of their choice, whether good, evil, chaotic, lawful, or neutral. They also have the ability to dispel the undead.
Thieves can wear only leather armor and may not carry shields, and can wield any type of weapon, including magical ones.
See also: Dungeons & Dragons, D&D, D&D races, D&D abilities, D&D races, Advanced Dungeons & Dragons
* These are rules given for D&D, not Advanced Dungeons & Dragons, which has some more particulars, especially concerning the higher levels, and more sub-classes.
Information taken from "Dungeons & Dragons," 1979, published by TSR Hobbies, Inc.