One of the most popular kicks is the round kick, sometimes referred to as a roundhouse kick. In Karate, it can be delivered with either the front or rear leg. What makes this technique so popular is that it comes from the side, so it is not seen by the opponent until it is too late. For this reason it is sometimes referred to as a “blind” technique.
Here is a good explanation of how the impact is delivered:
Here is instruction on how to deliver the kick from UFC Champion and Shotokan Karate black belt, Lyoto Machida:
Of course, no technique is 100%. Here is a good method of defending against the round kick. Notice however, that the defender needs to move into the attack, something that is often difficult to do in the heat of the moment.