the Shimano road groups (collections of parts meant to work together) are (from cheapest to best) 2300, Sora, Tiagra, 105, Ultegra, and finally Dura-ace.

on Mountain Bikes, its (again, cheapest to nicest) Tourney, Altus, Acera, Alivio, Deore, Deore LX, SLX, Deore XT, adn XTR. There might be some new specialty ones in there for DH bikes like Saint.