Mashups are the most revolutionary Music movement
since the hippies gave birth to FM radio in the late '60s
and filled our ears with engrossing segued music sets.

Technically, a music mashup is the not-so-simple matter
of playing two (or more) songs at the same time, manually
lifting the lyrics from one and the instrumentals from another.

Sociologically,

Psychologically, it is stunning to hear a well-known song yet
know you have never heard it before... not like THIS!

Here is the Wikipedia history and discography of mashups.

Next, for a clearer appreciation of the depth of this phenomenon,
go Google "+mashups +music" to find (over) 2,400,000 links!
featuring DJ remix talents and many do-it-yourself tutorials!

(Note: If you search just for "mashups" you get about 3,800,000 hits,
because the term includes video mashups (many on U-Tube) plus software
mashups where two+ different programs are combined to function as one.

Currently, Mashed music and video are still illegal artforms.
However, a legal solution was boilerplated decades ago, when
Rap artists began "sampling" snippets from earlier recordings.

EMI Music recently released an "official" Mashups CD
in Europe which sold out. Search for "Mashup" on Amazon.com
and you'll find a half-dozen complilation discs and ± 50 "legal" MP3s.