A mashup is a website or web application that seamlessly combines content from more than one source into an integrated experience.  Check out two 2 great mashups:   101World.Com and this site MASHUPS101.COM (did you know that this site you are reading now is a Mashup?).   

The etymology of this term almost certainly derives from its similar use in pop music where DJ's take the vocal track from one song and combine it with the instrumental track of another song resulting in an entirely new composition.  

Many people are experimenting with mashups using eBay, Amazon, Google, and Yahoos APIs. The increased use and popularity of mashups has increased with the emergence of Web 2.0, which is characterized by active user participation and interaction.

Much like how blogs revolutionized online publishing, mashups are revolutionizing web development by giving creative power to the masses. Many mashups are relativley easy to design with minimal technical knowledge, and thus custom mashups are being designed by unlikely innovators, utilizing data in creative and unique ways. While there are several useful mashups, many are simple novelties or gimmicks, with minimal practical utility.

Most mashups interface with and utitilize information from established companies and web applications.

It is postulated that mashups are the beginning of the end for traditional operating systems. While operating systems currently serve as a collection of APIs for organizing and storing files and applications, it is quite possible that in a few years all APIs will exist entirely online, and that operating systems will be replaced by browsers.

Mix06 A three-day Microsoft-organized event with the tagline "Mix the next Web now."

TechSoup's NetSquared Remix the Web for Social Change