Maycomb is a town where pretty much everyone knows everyone else's business--except when it comes to the Radley family. For the most part they stay shut up in their house and do not socialize with the rest of the town. Arthur "Boo" Radley has not been seen for many years, which allows the town's imagination and the imagination of the kids to run wild. Because they have no facts about Boo's whereabouts and activities, they make them up--they surmise that he sneaks out at night and looks in people's windows and eats live animals! Boo is different to them because he lives outside of the norms of their society.
Boo Radley has essentially been held prisoner by his religious zealot of a father since he was in his teens. Boo got in trouble with some other boys and was going to be sent to a reformatory, but Mr. Radley insisted on punishing Boo himself. When the story begins, Boo is in his early thirties and has not been seen in about 15 years, save the time he stabbed his father in the leg with a pair of scissors. So Boo has had no contact with the outside world since he was a teenager, and one could imagine what kind of effect that would have on a person.