So what is a Kettlebell and why is it so popular? We get this question all the time! Kettlebells exercises check all the boxes, cardio, strength and flexibility training. First of all Kettlebells are named very appropriately. They are a large cast iron ball shaped weight with a single handle. It resembles a cannon with a handle. It is used to perform many types of exercises ranging from weight lifting to power training exercises that combine cardio, strength and flexibility training.