Ghouls are the first enemies the Barbarian fights during the Diablo 3 demo video. Although individually weak, they attack in massive waves to overwhelm you.
Walking corpses are slow, but are tougher to kill than Ghouls.
Zombies are often seen with Walking Corpses, but are faster.
Some Walking Corpses simply turn into Crawling Torsos after you kill them
Skeleton archers harrass you with barrages of arrows. They are weak, but often hide behind Skeleton Shieldman to shoot from safety
Skeleton Shieldman carry enchanted shields which can block most physical damage.