Although the previous post's answer about "racial divides" is a key issue of the novel, I believe To Kill a Mockingbird is primarily a story about a young girl and her brother growing up--and confronting some very trying circumstances--in the Depression-era South. While the Tom Robinson trial serves as the main plot of the second half of the novel, the racial awakenings that Scout and Jem endure are only a part of their rapid rise toward maturity and their loss of innocence. Scout falls in love with Dill and begins to discover the feelings that a grown-up feels as well as the responsibilities that go along with becoming a lady during this time period in Alabama. She discovers first-hand the hypocrisy that exists in Maycomb, and how some "ladies" behave as no lady should; and she learns about tolerance, real courage, and humility from her father. She lives through the early years of believing her neighbor, Boo Radley, is some sort of murderous ghoul before recognizing that he is a kind-hearted man trying to show that he is her friend. Boo is ever-present if never seen, and Scout fantasizes about the day when she will finally meet him face-to-face. Little does she know that that day will come when she least expects it, presenting Boo with the opportunity to show that he has been keeping an eye on her all the time: He is not only a good neighbor, but her protector, coming to the rescue when she needs him most and saving her life by thwarting the plans of the murderous Bob Ewell. On the final page of the novel, she discovers that, like the character in the novel which Atticus had been reading to her, Boo was nothing like other people had described him.
"... when they finally saw him, why he hadn't done any of those things... Atticus, he was real nice..."
It is a novel about children growing up and of recognizing the faults and frailties of the people around them.
The key points to To Kill a Mockingbird have to do with the challenges of racial divides. A white lawyer represents a black defendant in a town that does not support the blurring of racial lines. Ultimately the book is about doing the right thing for people and attempting to be color blind when acting on their behalf. Atticus is trying to instill a moral compass and general understanding of good verses bad in his children and into the small town mentality around him.