Anyways, you enter the codes through the extras menu on the main screen.
Codes are usually only available for a couple hours, but some are for a certain amount of people. You can generally get them from the Borderlands Facebook page or from the twitter accounts.
They're useful to spend at pretty much any level, and it's generally a blue-purple quality you'll get from the chest for the level you're playing at. Not always helpful though...Last two times I opened the chest on my gunzerker I got a commando and siren mod, and then 2 corrosion/burn resist relics, a blue and a purple, and both un-needed. Was good cash though.
Karma is a way of showing other gamers that you don't have a life.showing that you are active means you have too many games