basically ric has it all down. but to sum it up, it can increase the performance of ur gpu by "overclocking". with a increase of heatings.
u are telling the card to perform at a higher rate then the stock settings.
just compare a gtx 285 to a gtx 285 superclocked . the SC version has been factory clocked to perform at a higher speed. the only difference from a factory clock to a self oveclock is that factory clocks are much more guaranteed performance increase. and is more stable than self overclock.
but be aware to be CAUTIOUS when overclocking. dont "over" do it by putting everything to the max. it takes time. u have to experiment.