Zombies and Skeletons
The difference between the two is that zombies are 'fresher' dead than skeletons and have not yet fully decomposed. They also retain some very limited aspects of their original selves and, without any outside influence, may wander back to places that had an impact upon their lives (often where they lived, or the place they died if they had been killed). Skeletons are far more mindless. They act with no purpose and tend just to wander at random though they often will not stray far from the place they were buried.
Being lesser dead, neither has any regenerative powers and so wounds will not heal. Zombies also progressively deteriorate physcially (and mentally, not that there was much to start with) until they reach the same stage as skeletons. They possess the same hatred of the living as almost all Undead and will attack on sight any living being. They will pursue a target until either it or they are destroyed (or until the pursued manages to give them the slip).
Being mostly mindless, zombies and skeletons are particularly favoured by necromancers as servants and soldiers: they are relatively easy to reanimate and control and there are plenty of eligible corpses out there...
Alignment (if any): evil - undead
Life expectancy: n/a