The turn undead button can work in two ways. Good and Neutral clerics, as well as paladins, use the ability to actually turn them (more on that later) while evil priests supposedly take control of the undead (haven't tried this myself though).
Undead 'live' because they are maintained by a certain power. Sometimes this leaks from the Negative plane, other times it is summoned by, for example, wizards. A cleric turning undead seeks to remove the energy that animates the bones. How succesful it is depends on how well trained the cleric is andon how strong the undead creature is (stronger ones have more enery to remove, or the energy is better managed).
In IWD, there are three different effects you can have while turning undead. A low level priest facing a high level undead creature can turn all hewants, but there'll be no effect. A high level priest turning a low level undead (like a zombie or skeleton) can instantly destroy it. Anything inbetween does affect the undead but won't destroy it. This is the most common effect, and the effect of such a turning is to make the undead want to stay away from the cleric, out of a fear-like desire not to be destroyed. In game, the effect is to have the skeletons flee.
The exact levels needed to destroy or 'scare' an undead creature depends on the creature itself as well, so I can't give you all the details from the top of my head [img]smile.gif[/img]
Oh, and please don't apologise for asking questions. That's what this forum is for, okay?
