Karma is one of the ways in which you can get a feel for the work that someone has done recently in QuickBuild.
Almost everything you do in QuickBuild helps you to build karma. For example: registering bugs, translating strings and answering support requests. Different types of work earn different levels of karma. Also, the amount of karma a particular action gives you fades over time.
Karma is only attributed for work done in QuickBuild and does not apply to items imported from other sources.