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 most associated with the more abstract and challenging strains of electronic music, jazz, and the modernist avant-garde of the mid-20th century.
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 (e.g., drone music or free improvisation), unusual instruments (e.g., the theremin, or Harry Partch's homemade instruments), extramusical sounds (e.g., musique concrète or Matmos), "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 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.