KarmaKarma serves as a player controlled reputation system.
You can use a player's karma as a rough guide to how "good" or "bad" they are as a player. In general, players with lots of positive karma are trustworthy and helpful, while you'll find those with lots of negative karma (or very low for their age) aren't.
You can give/remove karma from any message board post they have made. You will not be able to send karma to another player who is on the same IP, or network as yourself. This is to stop people using multis to raise their own karma.