What exactly is a counter tenor?

I just started singing and I think I'm a baritone. My range is from the F below middle C to the F above middle C. I can't go very high and all of my singing idols can hit really high notes. I recently found out that baritones can train themselves to become counter-tenors. I've done some research and I'm not really understanding what it is. I know they use falsetto to achieve their sound but I would like a detailed description using terms that an uneducated singer would understand. Thanks.