Reddit Karma is an accumulation of "goodwill" you receive when users upvote your posts or comments. It doesn't give you any overt influence and it cannot be cashed in for fabulous prizes, but a healthy amount of Karma on your profile alerts others that you add value to the community.

There are two types of Karma: one for links and one for comments. Both are displayed on your user profile in the upper right-hand corner and are to be worn as badges of honor.