Shorin-ryu is one of the major modern Okinawan martial arts. Said to have been founded by Sokon Matsumura during the 1800s, Shorin-ryu combines elements of the traditional Okinawan fighting styles Shuri-te and Tomari-te. Shorin-ryu is widely considered to be one of the two major modern styles of Okinawan karate, along with Goju-ryu, which is rooted in the other traditional Okinawan style, Naha-te.
Sokon Matsumura was a renowned warrior of his time; he has been called the Miyamoto Musashi of Okinawa. However, while he is often referred to as the "founder" of Shorin-ryu, he did not invent all the components the style, and perhaps didn't ever call it "Shorin-ryu" himself. It is quite possible that he synthesized his knowledge of Okinawan arts with Chinese fighting styles that he learned on his travels and taught it as a coherent system to some eager students, who subsequently refined it, labeled it, and passed it on. (Highlighting Shorin-ryu's Chinese heritage is the fact that "Shorin" is the Japanese pronunciation of the Chinese 少林, "Shaolin"; "ryu" means "school", or "style".)
Along with being a style on its own, Shorin-ryu is also perhaps the most influential single ancestor of modern Japanese karate. One of Matsumura's best-known students, Anko (or "Ankoh") Itosu became a great practitioner and teacher of Okinawan karate and developed the five Pinan kata, which are now taught not only in Shorin-ryu, but also in a wide variety of Okinawan, Japanese and derived martial arts. It is also believed by some that the first two Pinan katas were actually developed by Matsumura and the last three by Itosu. In addition, Itosu and another student of Matsumura's named Anko Azato were among the primary influences on a fellow Okinawan named Gichin Funakoshi. Funakoshi introduced his Okinawan martial arts to mainland Japan in 1922, and in subsequent decades was instrumental in developing what he termed simply "karate" or "karate-do" as a popular Japanese sport and art. (The style Funakoshi taught on mainland Japan is now called Shotokan karate.)
Shorin-ryu is generally characterized by natural breathing, natural (narrow, high) stances, and direct, rather than circular, movements. Shorin-ryu practitioners will say that deep stances are not important for powerful moves, and that only correct motion matters. In fact, Okinawan traditionalists often claim that deep, wide stances are a development of Japanese-styled karate, and are useful only for show or for building up your legs' strength in order to make a good strong throw.
There is not now, and perhaps has never been, a single unified school of "Shorin-ryu," although many dojos use the term for simplicity's sake. Some of the best known schools of Shorin-ryu include Shobayashi-ryu, Ryukyu Hon Kenpo, Matsumura Orthodox Shorin-ryu, Seidokan, Kobayashi Shorin-ryu (Shorinkan) and (Shido-kan), Matsubayashi-ryu, Okinawa Kenpo, and Sukunaihayashi (Shorin-ryu Seibukan), but there are many others, most with long and distinguished histories that trace back to Matsumura and his students.