What is special about a Mockingbird?

Many of you have been asking why the novel is titled To Kill a Mockingbird.  Answer the following questions, what exactly is a Mockingbird?  Why would someone want to kill this bird?  Then, make some predictions about what the significance of the title might be.  Please remember that most titles usually do not represent a literal meaning but a symbolic one.  Also, if you have read the book or have seen the movie, please do not give away any plot points or outcomes.