Who is the mockingbird referred to in To Kill a Mockingbird?

There are two "mockingbirds" in Harper Lee's To Kill a Mockingbird: Arthur "Boo" Radley and Tom Robinson.
In Chapter 10, the concept of mockingbirds is introduced when Atticus gives the children air-rifles and cautions them:

Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird.

Miss Maudie adds to Atticus's words: 

Mockingbirds don't do one thing but make music for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird.

Thus, the mockingbird becomes a symbol of innocence. 
Boo Radley
At first, Arthur Radley is thought to be a "malevolent phantom," but the children learn his story from Miss Maudie and gradually become sympathetic to him. When he leaves them little gifts in the knothole of the tree that the children pass each school day, Scout and Jem begin to perceive Boo as a real person, a person who does not bother anyone. Of course, the children realize what a truly good soul Boo has when he risks his life to save them from Bob Ewell.
Tom Robinson
Another person who is kind-hearted, charitable, and lacks malevolence is Tom Robinson. Tom stops on his way down the road to help Mayella Ewell with some of her chores, even though he knows how racist her father is. In spite of his learned fear of the white man, Tom enters the ramshackle dwelling in order to "bust up a chiffarobe" and bring in kindling for Mayella because of his sympathy for her. Rather than receiving thanks for his charity, Tom is wrongfully accused of raping the very girl for whom he has done so much. He is put on trial and convicted on no substantial evidence. When, in his desperation, Tom tries to escape from prison because he fears he will be hanged, he is shot seventeen times.
Not long after Tom's death, in his editorial in the Colored News section of the newspaper, Mr. Underwood "likened Tom's death to the senseless slaughter of songbirds [e.g. mockingbirds] by hunters and children."
Clearly, then, Tom Robinson and Boo Radley are metaphorical mockingbirds in Harper Lee's narrative.