Banned books

From Two Women Blogging, the Top 20 banned or challenged books in the US during the 20th century. The ones I've read are in bold.

I don't understand understand the concept of banning books, but in particular I don't understand why you'd ban the following books.

1. The Great Gatsby by F. Scott Fitzgerald
2. The Catcher in the Rye by J.D. Salinger
3. The Grapes of Wrath by John Steinbeck -read outside of school
4. To Kill a Mockingbird by Harper Lee
5. The Color Purple by Alice Walker
6. Ulysses by James Joyce
7. Beloved by Toni Morrison
8. The Lord of the Flies by William Golding
9. 1984 by George Orwell
10. The Sound and the Fury by William Faulkner
11. Lolita by Vladmir Nabokov
12. Of Mice and Men by John Steinbeck
13. Charlotte’s Web by E. B. White
14. A Portrait of the Artist as a Young Man by James Joyce
15. Catch-22 by Joseph Heller
16. Brave New World by Aldous Huxley
17. Animal Farm by George Orwell
18. The Sun Also Rises by Ernest Hemingway - tried to read
19. As I Lay Dying by William Faulkner
20. A Farewell to Arms by Ernest Hemingway