Karma is a system that keeps track of how many of your reports have resulted in a ban. This system does not award anything, and is merely a measure so that moderators can better detect whether a report was made in good intentions.
You gain 1 Karma point for every reported player who is banned, and you will receive a message upon the ban informing you that you have gained a point (and your current total). There is no way to see manually your Karma points.
Originally bad reports would also remove a karma point, but this has since been removed.