In most sports, you have to learn a few different skills. Tennis players, for example, need to learn different strokes like the serve and forehand. Basketball players learn how to dribble and shoot. In mixed martial arts, you not only have to learn different skills, you have to learn multiple martial arts.
Some fighting sports are fairly simple, such as boxing. You have two hands that you can hit your opponent with and sometimes you can grab them in a clinch. Other martial arts, like Judo, have a myriad moves that you can do to take your opponent down and force them to submit.
The martial arts vary in complexity and the style of fighting that takes place. Generally, the styles are broken down into striking and grappling. Boxing and kickboxing are forms of striking arts, while wrestling and jiu-jitsu use grappling techniques.
Striking Martial Arts
Performed standing, the striking martial arts are extremely quick and dangerous. Mixed martial artists use these when they're attempting to damage an opponent or knock them out.
Grappling Martial Arts
Unlike striking, which is quick and fairly violent, grappling is an attempt to gain the upper hand on an opponent by wrestling them to the ground and forcing them to submit, or tap out. Bending limbs or choking an opponent can force them to tap out, which means that you win the fight.