A kettlebell is essentially a spherical weight with a handle on top.  Kettlebells can be used in many different ways to give a trainee a good workout.  They can be used much like traditional dumbbells to work individual muscles and muscle groups.  The kettlebell gives a better workout to the stabilizing muscles of a trainee because the weight is slightly offset from the handle.  Perhaps the most valuable workout that can be performed with a kettlebell is the kettlebell swing, which works multiple muscle groups at once, and is great workout to improve core strength.