Experimental music consciously deviates from established musical norms (of a particular genre, or of music in general). Music of almost any genre can be considered experimental – but the term is often associated with the more abstract and challenging strains of electronic music, jazz, and rock. Avantgarde has its own tag.
Experimental music might explore new compositional techniques (e.g., John Cage's use of the I Ching, or Steve Reich's process music), new musical forms (but drone and free improvisation have their own tags), unusual instruments (e.g., the theremin, or Harry Partch's homemade instruments), extramusical sounds (e.g., Matmos, but musique concrète has its own tag), "extended technique" (playing instruments in unusual ways – e.g., Cage's prepared piano), microtonal scales, minimalism (e.g., lowercase music), unexpected stylistic fusions, abrasive timbres and rhythms, or experiments in any other aspect of music.
As such, "experimental music" isn't really a single style with a distinct sound. It's more of a sensibility: an effort to explore new musical frontiers, and to expand the boundaries of musical experience.
There is major potential for overuse of this tag, which can be thinned out via use of the various others mentioned on this page instead.