Uses all spells and skills available to said class group, and they can use them without fail.
Rogues- Encircle
Fighters- Huh, I think they just bash
Casters- Can cast icky icky spells
All I believe is purely based off of their DR/HR which is set REALLY high.
Anubis is busy writing a true Mob A.I. that will allow mobs to use proper skills/spells according to class groups (fighter, rogue, caster, etc.,) appropriate to their level. (hope I got that right Anubis).
As for Undead, no clue.
Animal means they can claw ur face off or something- Mainly it prevents gold from dropping from animals. Personally, I always thought that was the dumbest change made to mobs. If DnD says I got a treasure from killing a giant mountain cat- THEN I GOT TREASURE FROM A GIANT MOUNTAIN CAT. Give'us our gold damn it

Normal- I believe is just a general non skill/spell class- Cannon fodder really.