bad acting is pretty self explanatory, it is not portraying your character correctly, not using the right amount of emotion and feeling in your act and in your expressions and actions. i cant really think of any bad actors right now, but there are plenty. a good actor that i can think of from the top of my head is johnny depp, in every one of his films he does a spectacular job at becoming his character, using the right timing, expressions, actions etc. i think any actor who does a movie like american pie or some stupid crap like that, is a bad actor, yes they may do a good job of their character, but how hard is it really to play that kind of role, theyre probably already a bit like the character themselves, all they have to do is say the lines like they would normally to someone theyre speaking to, no skill in that.