Mawashi-geri (roundhouse/round kick) is a kick employed in Taijutsu, as well as several other Japanese martial arts. Roundhouse kick is a basic kick of many styles outside of Japan as well, including styles from Korea, where it is known as dolryo-chagi.
How to execute a roundhouse kick
Step One: Starting in sparring stance (body sideways), pick up your back leg.
Step Two: Turn your hips and simultaneously pivot on your other foot.
Step Three: Kick by snapping out the lower portion of the leg. However, keep in mind that even in kicks involving the knee, the power for the kick always comes from the hip.
Step Four: Re-chamber your leg and land the kicking foot in front so that you continue to move towards your target.
Roundhouse kick can also be executed with the front leg, following the same steps shown above only it is the front leg that is picked up rather than the back leg.