"Shh... Go to sleep."
In my opinion Jeff the Killer's story isn't that great. The writing is terrible, full of plot holes and a lot of things just don't make sense. However, its the sheer popularity of this character that gets him on here, and the fact that he is somewhat of an homage to classic slashers like Jason Vorhees and Michael Meyers.
Jeff's story basically describes his transformation into the murderous psychopath we know today. He and his brother had moved to a new neighborhood with their family and he'd gotten in a fight with some local kids, leaving them beaten and stabbed. Then next day, the kids attack him again, and a freak accident causes his face to be burnt. He then carves a permanent smile on his face and burns his eyelids off, and proceeds to murder his entire family. To this day he is still at large, so make sure you keep your doors locked.