There have been a few threads about this in the past, some more comprehensive than others. I think certainly the roles include:
1) damage dealer(s) (mandatory more than others, but doesn't have to be 100% specialist)
2) battlefield control (could be tank with reach, CC/debuff caster, etc)(not 100% necessary, but very nice to have)
3) healing, damage mitigation (buffs) and negative status effect removal (not 100% necessary, but nice to have. Can also be done via potions, items, etc.)
4) a way to deal with traps (can be a Log (TM), undead, sacrificial summons, or a rogue, etc)
5) face/negotiator (not always needed in all games)
6) Scout (not 100% necessary, but very nice to have.) (Can be stealthy class, caster with a familiar or divination, etc)