A round-off is basically the same thing as a cartwheel, but your feet land at the same time together.
A cartwheel is the most well known of all gymnastics skills!
In a cartwheel, your starting position is called a "lunge." One leg is in front, one behind, your arms are straight by your head, and your body should be facing forward (not to the side!). Your front leg is bent and your back leg should be straight. The next step is to put your hands down (side by side) in line with your front leg, and kick your back foot over your head, followed by the front foot. You should land in a lunge, with the leg opposite of which you started in front.