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.
What are you doing? You are supposed to be reading this tiny ad!
the new thread boost feature brings a LOT of attention to your favorite threads