This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
They are for different purposes. A StringBuffer is used when you want to insert/remove/splice/etc characters on the fly, something Strings can't do. A StringBuffer might be loosely thought of as an ArrayList of chars, since it can grow and shrink dynamically. A String can not change its size. HTH!