is music that isn't exposed to the public
by means of advertising
play and other media
Often closely related to independent music (or "indie"), since underground music often is marketed and distributed by independent, small non-profit record labels and organisations, or often by the artists themselves.
The internet has become extremely useful for the underground movement, since anyone can publish almost anything at an extremely low cost. In the long run, this will probably help independent artists market themselves without having to turn to a record company for (expensive) help to do so.
On the other hand, many people feel that an artist who becomes very popular, even though starting out in the underground has let down the underground movement by becoming too commercial. Often called a "sell-out". Good examples of this are Nirvana and Metallica.
Some typical underground music styles are club music, techno, extreme metal (like thrash metal and death metal) and punk.