CrossFit is a highly effective general purpose physical fitness program that achieves results through short, high intensity, infinitely scalable work outs that use a constantly varied mixture of functional movements. It is generally done in small groups of people who encourage and motivate each other to do their best, which results in some really great friendships and an amazing sense of community. Oh, and it is a lot of fun!
Okay, what does that really mean? This is a video from CrossFit HQ that explains.