Generally it is just a net score. The score is just incremented and
decremented
as each positive/negative vote is submitted. Some sites such as
www.perlmonks.org (let's try to get this back On Topic! :) ) use a
more elaborate algorithm though.
http://www.perlmonks.net/?node=Voting%2FExperience%20System explains
how perlmonks "Experience Points" are calculated.
So, there really is no one way and you may want to build something sort
of fun like perlmonks XP to help build a certain "community".