Theoretically, the trial of Tom Robinson taught some of the people in Maycomb that racism is wrong. The white people second-guessed their prejudices, and the black people saw a glimmer of hope.
Atticus Finch actually defended Tom Robinson, first of all. Although he was appointed, he made his best efforts. He did prove that Mayella and Bob Ewell was lying, that the crime never took place, and that Tom Robinson could not have physically committed it. When Atticus lectured the jury on racism, he was lecturing most of the town—because they were all present and watching.
[Some] Negroes lie, some Negroes are immoral, some Negro men are not to be trusted around women- black or white. But this is a truth that applies to the human race and to no particular race of men. (ch 20)
Atticus’s words did have some effect. The jury actually deliberated before returning a guilty verdict. Although they said that only Atticus Finch could have produced this result, it does not mean they just waited around. They actually look the time to talk about the facts.
"We're the safest folks in the world," said Miss Maudie. "We're so rarely called on to be Christians, but when we are, we've got men like Atticus to go for us." (ch 22)
The trial does affect the black people. Atticus finds presents from the black community, thanking him for trying. They give what little they can. They are thanking Atticus for truly representing them, and not just sitting back and letting Tom Robinson be convicted without putting up a fight.