Basically rate ups. Karma: By achieving a base karma above 3.00, you will move up a rank, and are now a Scholar. There are three main forum levels:
Scholar - 3.01-3.65 - the majority of posters fall into this category
Sage - 3.65-4.39 - an exceptional poster, one who consistently provides good information and interacts well with the community. Rare to attain.
Guru - 4.40-5.00 - This poster participates above and beyond the call of duty. Their word is unquestioned, and in their area of expertise they are unparalleled. it takes a rare poster indeed to reach this rank.
Administrators - The forum administrators also have a custom colored title.
A post may still be rated when it is at either extreme of the scale. for example, on a post that has been rated to 5.00 that receives another excellent 6.00 rating, that 6.00 rating does count towards the posters base karma.
And, Stars: The stars located under various people's usernames are related to how many posts they have. The more stars, the more posts. Stars are independent of the Karma system.
Edit: Guess I should tell you where I got this info from;
Markup Help Edited, Jan 8th 2009 6:16pm by Kirby