Banned books are books that are deemed inappropriate, obscene, offensive or generally unfit for classrooms or to have available at libraries due to their content. It is not illegal to read banned books, although it might be difficult to track down a copy do to the efforts of censorship. Books can be banned locally but not nationally, books that some schools have banned might be on the required reading list at others! A book being banned does not mean that it is without merit or value, just that other people do not agree with it or they personally may find it offensive.
Banned Books Week is an annual event that celebrates the freedom to read, it was originally launched in 1982 due to the vast influx of books being challenged across bookstores, libraries and schools. Banned Book Week typically takes place the last week in September and helps to promote the reading to read, and to read whatever you want. Many booksellers, libraries and schools use this opportunity as a teaching tool to raise awareness of freedom of speech and to celebrate authors and tough topics that banned books broach.
The reasons for the books are challenged are listed with the book entry (including the specific reasons by year), keep in mind that these are merely opinions and not actual facts, books are commonly challenged without actually reading the material in it's entirety or at all.