Systema is a russian martial art. It is not a sport, does not have a cometition or belt degree system like in karate or ju jitsu. There are general principles with which the systema practitioner learns to respond to an attack. These can be summarized into these four elements.Breathing is important for good oxigenation, and dealing with stress. Movement is the first form of self defence, it is better to just avoid a kick then to take the hit. With structure we mean the way someones body is organized, the way you anticipate an attack and the way your opponents body is organized. Relaxation is the fourth pillar of systema, a lot of techniques that people try to use on you become less efective when you recieve them in a soft manner.
This all does not mean that systema is inefficient, the creed is: recieve softly, but give back hard. A lot of techniques from other martial arts are usable in systema, as long s you use then in a systema manner.
Systema is a very good martial art when you want to learn to defend yourself effectively. This is of course something claimed by every martial art, but systema is unique because it teaches you your own personal style. We dont train for exams of cometitions. Because of this you can train in a frendly way with even the most advanced practitioners.