Experimental music relies on the concept of the composition itself having an uncertain performance.
The idea being that the composer no longer controls every aspect of the composition, that some parts are left as variables. That there is the possibility of two completely different performances of the same piece.
A very renowned piece is 4'33" by John Cage. Although no longer relevant in terms of today's music. At its time it was one of the first pieces of experimental music composed.
Other experimental pieces include the use of the audience in the very make up of the piece. So that each individual performance varies with the audience.
However the term 'Experimental Music' is often mis-used to define music that is, rather, not an experiment in itself. But rather breaks the musical rules and confines that have supposedly been imposed.
Artists such as Moby (see below) are not really experimental musicians. It can be argued that the music that they create was an experiment originally. As It was.
If anyone attempts to create music by melding and breaking genre's in a way that has not been done before, it is, in a sense, experimental. But it is not to be assumed that their music is experimental. It is instead only themselves who have experimented, and only once usually.
There are also bands such as Pink Floyd and Radiohead, who experiment many times over their musical careers. But they are not experimental musicians, nor do they create experimental music.
Their experiment is only that they are bending/breaking genre's. Not that they are composing music as an experiment.
An experimental piece has only the confines that the composer must be uncertain as to how the piece will sound, when performed, and performance to performance.