Mashup
Ask 10 self-proclaimed mashup developers what a mashup is and you just might get 10 different answers. That said, mashups (be they software or content-oriented) are most often characterized by the way they draw upon functionality and/or content from two or more sources (at least one of which is third-party to the developer) to produce a new creative work. For example, if a birdwatcher "mashed up" her own database of birdsightings (by GPS coordinates) with the functionality and content of Google Maps, the resulting visual presentation where the locations of birdsightings are interactively overlayed on top of a map within the context of a Web browser could be considered a mashup.
Web Mashup = API [A] + API [B]
Mashups could easily involve the merging of multiple sources, none of which the developer had a hand in developing (and, in fact, this is probably the most common form of mashup). In some cases, a mashup gives its visitors an opportunity to add their own data to it. An example of such an approach might be a mashup where cell phone users can plot the location of any not-spots they find (locations where their cell phones have no signal). The result is essentially a customer-developed coverage map that could be more trustworthy than the coverage maps provided by the various wireless carriers. A great index that includes all types of mashups is webmashup.com
To facilitate the use of their content and functionality, many Web site operators provide application programming interfaces (APIs) to make that content and functionality more easily accessible to developers who often use Javascript and a technique known as AJAX to build their mashups. In addition to keeping an index of those APIs, webmashup.com also charts their popularity based on the number of mashups that use each one.
A mashup is a website or web application that uses content from more than one source to create a completely new service. This is akin to transclusion.
Content used in mashups is typically sourced from a third party via a public interface or API. Other methods of sourcing content for mashups include Web feeds (e.g. RSS or Atom) and JavaScript.
Much the way blogs revolutionised online publishing, mashups are revolutionizing web development by allowing anyone to combine existing data from sources like Amazon.com, eBay, Google, Strikeiron, Windows Live and Yahoo! in innovative ways. The greater availability of simple and lightweight APIs have made mashups relatively easy to design. They require minimal technical knowledge and thus custom mashups are sometimes created by unlikely innovators, combining available public data in new and creative ways. While there are many useful mashups, others are simple novelties or gimmicks, with minimal practical utility.
Advocates and Supporters of Web 2.0 applications claim that mashups exemplify this new movement with their active user participation and interaction.