I figure everyone has a least seen the movie starring Gregory Peck but just in case here is what I told my four year old about the book.
It's a book told by a woman about her life when she was a girl. There is her dad, Atticus and her brother, Jem, their maid Calpurnia and a summer friend, Dill. It is a few years of their life in rural southern Alabama. It is a book about how to live in a way that treats all people with respect.
This was Harper Lee's only published book. Her writing was plain, witty, and poetic all at once. I highly recommend reading the book as well as seeing the film. Apparently Ms. Lee was quite pleased with the screen play and became good friends with Gregory Peck and his family so they must have gotten it right.