I don't believe a kettlebell training session is complete without Swings. Why?
Though they are considered a rather basic move, Swings remain one of the most powerful tools in the kettlebell arsenal. They strengthen the oft-neglected posterior chain and help build hip power and mobility. And they are simply fantastic for conditioning. As Chris Lopez explained in a great post on KettlebellWorkouts.com about the effectiveness of kettlebell swings on cardiovascular conditioning. He mentions research that revealed...
Continuous kettlebell swings can impart a metabolic challenge of sufficient intensity to increase Vo2max. Heart rate was substantially higher than Vo2 during kettlebell swings. Kettlebells provide a useful tool with which coaches may improve the cardiorespiratory fitness of their athletes.
I can vouch for the effectiveness of Kettlebell Swings for conditioning. My JKDU/MMA for the Street instructor test involved several hard rounds of fighting. To prepare, in the weeks before the test the only workouts I did involved Swings, burpees, bag work, and the occasional swim. This helped immensely, improving my performance and keeping me from getting overly gassed during the test.
If you want to see a good, simple demonstration of the Kettlebell Swing, check out this video by the KB Queen herself, Lauren Brooks: