Both Tom and Boo are alienated outsiders in the story itself. Boo, because so much community gossip surrounds his presence, and Tom, because he is wrongfully accused of the rape of Mayella Ewell.
Each represents castigation from society, and each has his own "cross to bear" within the Maycomb County community. Neither is respected, and both are kept outside the boundaries of "proper" society within Maycomb. As it turns out, both characters are vital to the story's plot events, with Boo saving Jem and Scout near the end of the story, and Tom being innocent of the charges which he faces.
In the novel To Kill a Mockingbird, both Tom Robinson and Boo Radley are imprisoned for crimes they have not committed but for which most of the town believes they are deserving. Tom, a black man, is believed guilty because Bob Ewell, a white man, accuses him of raping his daughter; and in 1930s Maycomb, the word of a white man always takes precedent over that of a black man. Boo Radley is relegated to a life of seclusion within his own house thanks to his father, who decides that this is better for his son than a reform school or mental hospital. Despite the minimal nature of his crime, he is serving a life sentence nonetheless. The two plots of TKAM--Jem and Scout's attempts to get a glimpse of Boo, and the Tom Robinson trial--come together in the final chapters when Bob Ewell attacks the Finch children (in retribution for Atticus Finch's strong cross-examination) and Boo comes to their rescue. At last, Scout sees Boo, and Tom Robinson's accuser is killed.
The most obvious.... They are the Mocking birds.
Both are outcasts from society.
Tom is a black man living in an era when being black meant being less then, not believed an outcast. (He is also handicapped). Yet despite all, his actions always show a very good person making the right choices. In doing the right thing he is being punished and in prison.
Boo Radley is a social outcast, because of misconceptions and problems he is thought of as a monster (thought to be mentally handicapped). To all he is the town “Boogie man” which is a label society has placed on him. Again His actions always show the heart of a good person making the right choices. He is the protector of the children ...The exact opposite of his public perception.
The title of the book is to “Kill a Mocking Bird”. Boo and Tom are the Mocking birds.