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.
I also recommend that when feeling the urge to ask "what is a...?" that your first response should be to use a suitable search engine. Chances are good that someone has already published a very good definition of the term in question. You will save yourself the time of waiting for a response, and the time of others from repeating information that already exists elsewhere.