Necros have a skeleton rogue (melee dps), tank, and skeleton mage (ranged dps).
Mage is pretty similar, Water pet is ranged dps with an interrupt I think.
Rangers get a boar (tank pet), dire wolf (dps), blood raptor (AE melee dps).
Beastmaster only gets one pet, but you can buff it to change it into tank or dps.
All pets have greater versions after ~20-24 points spent in the soul that scale past level 30 and have additional abilities.