i can't understand what turn undead does, can someone please help me?

Eh... yeah. You turn it on, and undead in the vicinity go boom. Not all the time, but sometimes, depending on the level of the cleric. If it's an evil cleric, they don't go boom, they get dominated.

You have to watch out when using an evil cleric (Viconia, for example) around non-party allies. Since an evil cleric can effectively dominate undead, they may attack your own allies, which will end up pissing those allies off and possibly cause them to attack you.

I hate the evil version of turn undead, to tell you the truth. All it does is make the undead go non-hostile so my party members don't attack them, but the stupid bloody things keep attacking us by default. That means I end up with another group of idjits to order around. I end up killing them anyway, so I'd prefer they just dropped dead in the first place. That's the one and only thing I don't like about Viconia, apart from her violent outbursts and excessive use of the words 'lowly male'.

I love turning undead. Watch them buggers run while getting thwacked upon by all sorts of missles weapons. Just one of many cleric abilities that make them more than just "healers".