Kettlebells are used for exercise and training. They look like a cannonball with a handle, come from Russia, and provide a great workout focused on whole-body exercise (rather than muscle isolation), with great benefits for strength, cardio, stamina, and flexibility. Unlike regular weights, the kettlebell’s center of mass is extended away from the hand which is optimized for a variety of different movements including swinging.
I specifically like exercises that work the entire body. The kettlebell does that and allows a continuous routine of various exercises without having to stop and change equipment. Moreover, you can do quite fine with a single kettlebel. You can start with a 1-pood (16.6 kg, or 35 lb) or lighter kettlebell and for many that will be enough. These weights take up little room, will not break down or wear out, and require no batteries.
[The founders of Cross Fit have published a useful guide for proper kettlebell swinging form.-- OH]