Most games have very clear delineations between good and evil. The bad guys wear black, and the hero fights for the virginal princess on behalf of all that is right and just. The difference in Fallout 3 is that the world has already gone to hell. Good or bad, the people that had the misfortune of surviving nuclear war are now just trying to eke out an existence. Maintain a shred of humanity and try to help others and you'll be rewarded with Good Karma, work only for self gain and your bad karma will pile high. The choice is yours and the consequences are drastic.