Boo Radley is a mysterious character. We know that he is a neighbor of the Finches; we know that he seldom comes out of doors; we know that people think there is something "not right" about him. Miss Stephanie tells the children about the a story that Boo once stabbed his father with a pair of scissors. Another rumor was that rather than send Boo to an insane asylum, the Radleys kept him locked up in his own house.

As the story progresses, we learn other things about Boo. Boo is the one who leaves presents for the children in the knothole. On the night of the fire at Miss Maudie's house, Boo is the one who puts a blanket around Scout's shoulders to keep her from shivering. And of course Boo is the one who ultimately saves Scout from Burris Ewell.

It's not clear what is different about Boo, but the book shows him to be a kind and courageous person.

Towards the beginning of the novel, Jem and the other children believe the many unfounded rumors about Boo Radley, which portray him as a malevolent, nefarious individual. Many of these rumors are spread by the neighborhood scold, Miss Stephanie Crawford. In Chapter 5, Scout asks Miss Maudie some questions about Boo Radley, and the reader gains insight into the reclusive life of Boo. The reader learns that Boo's real name is Arthur and that he is still alive. The reader also learns that Boo was a polite child and his father was a strictly religious man. Miss Maudie tells Scout that Boo's father was a "foot-washing" Baptist who believed that any type of pleasure was considered a sin. As the novel progresses, the reader learns that Boo is a compassionate, sympathetic individual who cares about the Finch children. 

Boo Radley was primarily known to those in Maycomb by the rumors about him.  Miss Stephanie Crawford was notorious for spreading sensational rumors about Boo.  Miss Stephanie told the children the story about Boo Radley and the scissors:

Boo was sitting in the livingroom cutting some items from The Maycomb Tribune to paste in his scrapbook.  His father entered the room.  As Mr. Radley passed by, Boo drove the scissors into his parent's leg, pulled them out, wiped them on his pants, and resumed his activities (To Kill a Mockingbird, Chapter 1).

Stories like these created fear in the minds of Scout, Jem, Dill, and other Maycomb children.  There were few people who offered any real information about Boo Radley.  Atticus was sensible about Boo, but he preferred not to talk about the Radley family.  Scout asked Miss Maudie for information, and she shared what she knew.  Scout asked if Boo were still alive, and Miss Maudie told her that yes, he was.  She also informed Scout that Boo's real name was Arthur.  Boo stayed in the house all the time, but Miss Maudie recalled him as a young boy.

Later in the novel, it was revealed that Boo Radley was kind.  He mended Jem's pants and covered Scout with a blanket on a cold night.  He even rescued the children from Bob Ewell.

Needless to say, most of the information Jem and Scout receive about their unseen neighbor Boo Radley--that he "peeped in windows," froze azaleas with his breath, mutilated animals and poisoned pecans--is merely speculative gossip spread by the "neighborhood scold," Miss Stephanie Crawford. We do know that Boo was arrested with some of the Cunningham boys from Old Sarum and

... came before the probate judge on charges of disorderly conduct, disturbing the peace, assault and battery, and using abusive and profane language in the presence and hearing of a female.  (Chapter 1)

Arthur Radley Jr. was released into the custody of his father,

... and Mr. Radley's boy was not seen again for fifteen years.  (Chapter 1)

Boo was next seen again after he apparently stabbed his father in  the leg with a pair of scissors, but old Mr. Radley "insisted that Boo not be charged with anything." Since

The sheriff hadn't the heart to put him in jail alongside Negroes... Boo was locked in the courthouse basement.  (Chapter 1)

Though Scout wonders if Boo is still alive, she is assured that he is since no one had seen his body carried out. Miss Maudie tells Scout that she remembers Boo speaking "nicely to me, no matter what folks said he did. Spoke as nicely as he knew how."

Most everything else about Boo is strictly speculative in Part One, including whether it was Boo that Scout heard laughing after she crashed on the Radleys' front steps; whether it was actually Boo's shadow the children saw on the Radleys' back porch; or whether it was Boo who placed the blanket upon Scout's shoulders on the night of Miss Maudie's house fire. In all three cases, it was probably Boo, but as always, the "malevolent phantom" was never seen.