As a karateka, I feel obliged to explain a little upon translator-san's not really correct, or rather, very simplistic explanation.
Basically, karate is a traumatizing, yet defensive (in its philosophy) martial art with emphasis on learning to utilize muscles in your body as good as you can, which would result in impulse forces. The uniqueness of karate is the way the fist strike is handled. When we strike in karate, we use the whole back muscles, contrasted to the usual usage of shoulder and upper back muscles in most other martial arts. It also has usage of legs, and depending on the style, can be from directly traumatizing (Kyokushin-kaikan) or semi-grappling with various technics (Gōjū-ryū).
According to the legend, karate was developed by Okinawan peasants to defend from Japanese invasions. That's why proper Okinawan karate goes hand in hand with Kobudō, martial art of usage of different weapons like sai and bo, basically field work tools originally. Now, Kobudou isn't really popular in mainland Japan, but so is the traditional karate. Meaning, Japanese karate is different from the original okinawan one. Though, the difference is more in the approach of teaching and passing the knowledge, than in the technic itself.
Actually, what's most likely is that karate was developed from several Chinese martial arts by Okinawan nobility. Peasants didn't really had neither time or energy to develop it, since they had to field all day.
About styles popularity, there are two most prominent ones - Gōjū-ryū and Shōtōkan, the last one making an ephasis on big powerfull blows, and tailored more for large body types that exchange speed to force. The story behind their spread is quite curious, but basically after a lot of straggle Gōjū-ryū ended up being taught in classical dojos and Shōtōkan ended up being used in Japanese universities, which transformed it to, though very popular (since taught in universities and later schools), yet extremely rigid style which preferred a one "correct" way of doing everything instead of a more classical approach of the "correctness" being depended on the situation and focus on the overall body usage, not little things like fingers positions.
So we have a situation when, in Japanese classical dojos the most prominent style would be Gōjū-ryū, balancing between agility, force, and speed, and in the media world the most popular would be Shōtōkan.