You know, I think it might be good for us to have a F.A.Qs topic for this forum. ^_^
As Melly said, this question has been answered numerous times -- a few good examples of the replies can be found
here and
here (a bit of info on how to edit them in the second topic, too). If you don't want to read through those topics, I've created a few different versions and posted them over at RPG-M,
here. :)