Generally, a BackGround is a couple of paragraphs of text, that you write down to help you think about your character before they are played. It may contain a PlotHook or two, and should generally give your character a reason to be getting involved in the TreasureTrap? world. Sometimes your BackGround will contain a DeepDarkSecret, in which case, don't put that bit on your wiki page.
Snapdragon thinks that all good backgrounds should contain at least one of:
"There was a man, a man from North. Son of a Demon and a Dragon, he led a contented life, until one fateful day. On this day, he was hunting in the woods, until he saw a column of smoke rising from his village. He returned home as fast as possible, just in time to see a combined Orc, Goblin and Drow raiding party riding away over the hill. He ran through the village to his home, and found his father lying in the mud, still alive, but just barely. 'Son, take my sword, and go to the south to seek your brother...'"
Androidkiller is amazed that Snapdragon didn't try to fit in the vampire, werewolf or catgirl bit into that :P