It is based on your activities, some are worth more than others. And you can even get a deduction, if you edit a post you'll lose five points. Rather contra-productive when it comes to correcting spelling mistakes 🙂 !
Under the red "My Stuff"-tab you'll find a Karma-button, if you press that one you will see your karma point history.
I'll have lunch with Martin on Tuesday, I'll check if I can trade in some of my points for a free lunch then and I'll let you all know if it is possible 🙂 !
Regards
Magnus