Kettlebells are made of Cast Iron balls with a handle attached, weighing anywhere from 5 to 100lbs. The Kettlebell's weight is not distributed evenly like it is with a dumbbell. This creates the need to counterbalance and stabilize your body during certain movements making it great for core strength, balance and coordination.
One of the greatest benefits of the Kettlebell, is that it is always a full body movement. When you pick up a dumbbell you often just muscle it to the position you like. With a Kettlebell, you actually use the transition from the floor to where you want the Kettlebell to go as a part of the movement. For example, if you want the Kettlebells on your shoulder to do a shoulder press or thruster, you must first swing the bell from the floor and do a clean to get it to the shoulder position. So even though you were doing a upperbody movement, you needed your full body to move the kettlebell to position.
Other benefits of Kettlebells:
They can provide dynamic movements, such as cleans, snatches, and swings
Provided coordination and agility
Combines strength and cardio
Better posture and alignment
Improves functional strength
Its time effective (full body, quicker workout)
Versatile and creative
Protects athletes from injury (working on deceleration of movements instead of stopping mid movement)