well, a moderator, for what I know, has to check out the site very often and read all the messages on his part of the forum.

He or she has to be some kind of "expert" in the field that he is moderating, so that questions don't stay unanswered.

As for differences, well, you can see that some guys don't always have good intentions, so someone has to be able to delete or edit the post that might be offensive.