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.
Read about how QuickBuild calculates karma.