This distress signal was first adopted by the German government radio regulations effective 1 April 1905, and became the worldwide standard under the second International Radiotelegraphic Convention, which was signed on 3 November 1906, and became effective on 1 July 1908. SOS remained the maritime radio distress signal until 1999, when it was replaced by the Global Maritime Distress and Safety System. SOS is still recognized as a visual distress signal.
The SOS distress signal is a continuous sequence of three dots, three dashes, and three dots, with no spaces between the letters (notated by the overbar). In International Morse Code, three dots form the letter S, and three dashes make the letter O, so "S O S" became a way to remember the order of the dots and dashes. In modern terminology, SOS is a Morse "procedural signal" or "prosign", and the formal way to write it is with a bar above the letters or enclosed in angle brackets: SOS or <SOS>.
In popular usage, SOS became associated with such phrases as "Save our Souls" and "Save our Ship". SOS is only one of several ways that the combination could have been written; IWB or V7, for example, would both produce exactly the same sound; SOS is just easier to remember.
According to wikipedia