A "loop" is a sound that is designed to be played over and over as background music or part of a sound clip. Mostly, loops consist of music that seamlessly goes together or cool little drum riifs - stuff like that. In general, you wouldn't use a loop by itself, but you would use it as part of a larger intro/outro or sound bumper.
There's a ton of free clips on the web. Bless the providers of such clips, but they're sometimes hard to deal with because of varied formats or quality levels. I have some good news, but you have to act fast.
Izotope, a producer of loops for Garage Band 2, normally retails their XRB loopkit for $49 US. For a limited time you can get a sample XRB loop kit for free and the full deal for $29. I don't know about you, but free is myyyy budget!