A mash-up is (usually) a bootlegged music track with no originally composed content, restructured from several - often disparate - songs, frequently to humorous effect.
For example, if you were to take the pounding piano chords from Stay Up Late by Talking Heads, loop them, and run an a cappella Eminem rap over the top, perhaps throwing some Britney Spears in at the chorus, you will have created a mash-up. But a truly great mash-up requires as much skill and dedication as an original track- you need to be able to identify complimentary tracks and sections of tracks, and craft them in a worthwhile fashion. One of the finest mash-up artists to be found on the web (where else?) is dsico. At time of writing, he can be found at http://www.4trak.net/dsico/. Among his classics are Heart of E, a cocktail of Missy Elliott and Blondie, and Dirty Bottle, pitching Christina Aguilera against Sonic Youth. Free downloads await.
Modern sound editing software puts the mash-up within every computer owner's reach, and the Internet serves as a powerful distribution / propaganda channel.
In the UK, the mash-up has achieved some chart succcess, particularly due to the producer Richard X, who has applied the technique to releases by Sugababes (using Gary Numan's Cars) and Liberty X (utilising Being Boiled by the Human League in a cover of Ain't Nobody by Rufus and Chaka Khan). Predictably, this led to something of a backlash in the "underground".