What
The Karma System is a good way to show the reputation of members.
If you think it was a good, helpful post, shoot the guy/gal some karma!

How
To the right of the post, there's a PLUS SYMBOL and a MINUS SYMBOL. Hitting the plus-symbol is a way of saying thanks, and this adds 1 to the posters Karma.
The minus-symbol should be avoided at all costs, but if it was a really, REALLY worthless and perhaps even rule-breaking post, hit minus.

PLEASE DO NOT VOTE MINUS JUST BECAUSE YOU DON'T LIKE THE GUY
Same with +, do not karma because of the poster, karma the POST.
It's not unusual that people go all like "I hate this guy, de-karma!", but please, DON'T.

PENALTIES FOR MISUSING THE KARMA-SYSTEM
If you in any way misuse the karma system, minusing or plussing posts WITHOUT REASON, you WILL have your karma lowered by the admins. Repeated offence will also mean WARN or BAN.
Whenever asked, you should be able to MOTIVATE the KARMA.

Guidelines
It's not needed, but good to add a post afterwards.
Write your Motivation to the Karma and then +Karma or -Karma.

E.G: "This was a nice tutorial, you explained the variable stuff in a good way which I was able to understand. +Karma"