In a sense, it could be argued that all music is experimental, and that it always has been. Since the beginnings of our species, human beings have been experimenting with ways to make music and song, and ways to reproduce and guide our soundmaking activities. Today, in the early days of the 21st century, Experimental Music has come to mean something more specific.
Some time in the 1950s musical experimentation seems to have split into two different streams. A composer’s stream and a performer’s stream. Both of these streams were important parts of Fluxus from the very beginning, and both composition and pure performance remain important today.
I recently wrote a very brief history and discussion of this on The Free Jazz Forum, a web based community for musicians working with Free Improvisation and Experimental Music.