Well I'm no expert but... A kettlebell is a lump of metal like a cannon-ball with a handle on. I believe they were mainly used by eastern block athletes who lacked the funding of the western countries. They were cheap, portable, and basic but highly effective conditioning tools. They are now used by the military and martial artists all over the world and there is actually a russian sport of kettlebell lifting. KB training is based around olympic style lifts like the power snatch which are performed in high reps, and strength lifts like the military press which are perfomed in low reps. The vast majority of the drills work the core stabiliser muscles and supporting muscles of the back and don't isolate muscle groups in an artificial way gym machines do. Some people liken it to yoga with weights. I like to think of it as calithenics with resistance. I'm new all to it but in the six weeks so far I've been getting superb results.