Good question since this is something I have switched "on" but haven't messed with too much yet.
The User Reputation system allows board members to leave comments about one anothers' posts, and thereby contribute to their overall 'reputation'.
On everyone's post, you will see a little "scale" icon. Click this and you can approve or disapprove with another persons post. You can leave a comment on this as well. You may view these comments (one's made towards you) in your user CP.
There are titles for a variety of reputation levels, and users are given a reputation title when they reach a certain reputation level. Most everyone starts off at 10 - I have increased the numbers on some of the oldtimers by the way
This is something that I belive will enhance the quality of the postings and I trust it will not be abused with "you suck" and these sort of comments. Unless of course the person really sucks