Here's been my take...
WARRIOR: Primarily concerned with dealing out and soaking damage in combat. Often had high hp, decent AC, and good attack bonus. Options included Fighter, Ranger, Paladin, Barbarian.
CASTER: An arcane spellcaster. First role is nuking (doing spell damage to multiple foes, or creating surrogates to fight) with a secondary role as "utility" caster (movement, illusions, divination etc). Wizard, Illusionist, Sorcerer are common examples.
HEALER: Typically a dvine spellcaster, primary concern is keeping the group healthy via hp renewal (cure) and negative status removal (removing disease, poison, death, etc). Secondary Utility role as well. Clerics and Druids often have this role.
EXPERT: Fills one or more skill roles: Scouting (stealth-based), Face-man (social skills), trap detection (find/remove traps) Survival (wilderness skills) and/or Access (opening locks, etc). Secondary role is skirmisher/flanker. Rogues, Assassins, Bards, Rangers, and Monks are all good Experts.
The roles could shift a bit (rangers could be a tank or an expert, depending on the build of the ranger/party, likewise bards and paladins could serve as backup healers or druids as casters or even tanks!) Typically, it was a good idea to fill all four roles, ideally they should be filled with the core 4 classes and other classes taking the role of second X or 5th man.