Ghouls appear to be the basic cannon fodder zombie of Diablo III. They are blue-skinned undead, too stupid to use weapons, but fleet of foot and quick of movement. They can be seem swarming the Barbarian in the beginning of the WWI 2008 gameplay movie, and die almost as fast as they race to battle (which is quite quickly).
Ghouls are an undead monster, but are nothing like slow shambling zombies; the ghouls are fast and uncanny, running hunched over in a bestial, almost quadrupedal style. They climb quickly and leap out at the player from all angles, scaling a huge wall to get into the action in the WWI 2008 gameplay movie.