If you've never heard of a kettlebell, it is what it looks like - a round hunk of metal with a handle on it. The magic of something so simple is that so much can be done with it. In all honesty (Greggles can testify to this), if you are going to purchase one piece of equipment to help you get in shape, its the cannonball with handle.
In the meantime, I will leave you with an overview instructional video from Pavel Tsatsouline. Even though the video is a bit cheesy (he plays up the Russian "comrade" schtick) and lo-fi, he is respected worldwide as one of, if not the top kettlebell instructors in the world. Enjoy the video comrades!