The basic premise of MMA (mixed martial arts) is simple. You stick two combatants in an enclosed area, and see who emerges victorious.
In my opinion, hand-to-hand combat is one of the purest forms of fighting. It's the truest test of a warrior. The only weapon you have at your disposal is your body. Anyone can win a fight with a gun.
Unlike boxing, which is confined to stand-up punching, and wrestling, in which striking is prohibited, MMA includes virtually all disciplines of martial arts. It essentially takes the best parts of the various martial arts and puts them to use in a (somewhat) realistic situation.
MMA is not the "anything goes" sport it once was. There are rules, regulations and weight classes, which ensure that the fights taking place are as fair as possible. You won't see unfair strikes such as eye-pokes, and you will usually see fighters of the same size/weight squaring off.
As Royce Gracie showed us, sometimes even size doesn't matter. With Brazilian jiu-jitsu, we learned that technique can triumph over size.
There are three basic elements that a mixed martial artist must master: Stand-up striking, grappling in the clinch and ground grappling. After all, these are really the only places that a fight between two unarmed opponents can go.
Now, all of this brings us to the martial artists themselves. In this day and age, we've come to see that mixed martial artists are some of the most well-trained and physically impressive athletes in the world. You must not only train your body, but also your mind. You must not only keep your body in top physical condition, but you must have knowledge of a vast array of techniques, as well as the skill and grace to execute them.
At the highest level, we have fighters like Anderson Silva—guys with transcendent skills. If you watch his fights, you'll see him bob and weave as he anticipates his opponent's strikes, then you'll see him retaliate with devastating pinpoint accuracy. You'll see him knock people out with a jab while backpedaling (Forrest Griffin), and you'll even see him incorporate new techniques (to MMA)—the front-kick knockout against Vitor Belfort.
The thing about mixed martial arts is that there is no preconceived notion of what it should be. Any discipline can be absorbed and adapted to MMA. Really, the only criteria is that it should be efficient and effective (and nowadays, entertaining). Because MMA is so malleable, it is always evolving. Therefore, its practitioners are some of the baddest men on the planet.