Bear Form:
Lvl 10
There is a quest that you need to do and you get that quest from a guy in the Cenerion Enclave Druid Tree at Darnassus.
(Increases your strength and armor)

Aquatic Form (Seal)
Lvl 16
Long, long quest given to you from the same guy in the Druid Tree that you got your bear form from.
(Increases your swimming speed by 50% and lets you breathe underwater)

Cat Form:
Lvl 20
Buy it from a Druid trainer anywhere for 9s.
(Increases your prowling and stealth abilities)

Travel Form:
Lvl 30
Buy it from Druid trainer.
(Increases your outdoor speed by 30%)

Dire Bear Form:
Lvl 40
Buy it from Druid trainer.
(Increases armor and strength)

A bear a dire bear a cat a seal and thats it i think