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.
This is a simple "what is ..." question. Such questions are answered the quickest and the best by doing a search instead of asking the question on a forum. This question has been asked many times before.
I literally copied your question into Google and it found approximately 28.300 answers for me in a fraction of a second.