Tom Robinson and Boo Radley are the two primary human mockingbirds of the novel--innocent men who are accused of crimes they did not commit. Boo becomes the most notorious character in Maycomb thanks to the mean-spirited gossip that paints him to be a nocturnal ghoul who stalks neighborhood pets, peeps in windows, and poisons pecans. There is no proof that Boo has committed any of these acts, but the gossip persists, thanks in part to neighbors such as Miss Stephanie Crawford. It's no wonder that Boo doesn't want to come out of his house considering the fear and hatred he would feel from the townspeople if he showed his face in public. Instead, Boo chooses to remain a recluse, saving his few trips into the outside world for acts of kindness, such as those he displays toward Jem and Scout--particularly on the fateful Halloween night when he stops Bob Ewell from his completing his murderous attack on the children.
Tom Robinson, meanwhile, is believed to be guilty simply because he is black and because his accusers are white. Atticus recognizes long before the trial what the verdict will be. In 1930s Alabama, a white man's word is always believed over that of a black man's, and no all-white jury could possibly be expected to take Tom's word over that of the Ewells. The fact that Tom is an honest, humble family man makes no difference: Even though the Ewells "have been the disgrace of Maycomb for three generations," they are white--and their skin color trumps the truth told by a black man.