In technology, a mashup is a web application that combines data from more than one source into a single integrated tool; an example is the use of cartographic data from Google Maps to add location information to real-estate data from Craigslist, thereby creating a new and distinct web service that was not originally provided by either source.
Mashup originally referred to the practice in pop music (notably hip-hop) of producing a new song by mixing two or more existing pieces.

Overview

Content used in mashups is typically sourced from a third party via a public interface or API, although some in the community believe that cases where private interfaces are    used should not count as    mashups. Other methods of sourcing content for mashups include Web feeds (e.g. RSS or Atom), web services and screen scraping. Many people are experimenting with mashups using Amazon, eBay, Flickr, Google, Microsoft, and Yahoo APIs, which has led to the creation of the mashup editor.

Quotes

a lot of talk about Web 2.0, web mashups, Ajax etc., which in my mind are all facets of the same phenomenon: that information and presentation are being separated in ways that allow for novel forms of reuse.
—Sho Kuwamoto
 

The mash-up part of this equation, is the offspring of an environment where application developers see it in their own selfish interest to facilitate the creation of integrated, yet highly derivative application hybrids by third parties, something they do by providing rich public APIs to their user base.
—Mark Sigal
 

(Popfly is) a (mashup) tool for end users, not necessarily codeheads.
—Steve Ballmer

Types of mashups

Mashups currently come in three general flavors: consumer mashups, data mashups, and business mashups. The most well-known type is the consumer mashup, best exemplified by the many Google Maps applications. Consumer mashups combine data elements from multiple sources, hiding this behind a simple unified graphical interface. Other common types are "data mashups" and "enterprise mashups". A data mashup mixes data of similar types from different sources (see Yahoo Pipes), as for example combining the data from multiple RSS feeds into a single feed with a graphical front end. An enterprise mashup usually integrates data from internal and external sources - for example, it could create a market share report by combining an external list of all houses sold in the last week with internal data about which houses one agency sold. A business mashup is a combination of all the above, focusing on both data aggregation and presentation, and additionally adding collaborative functionality, making the end result suitable for use as a business application. A Telecom mashup is a telecommunications service where service elements come from more than one source and are combined into an integrated experience. For example, one could get the base service from company A, a ringback tone from company B, a voicemail service from company C, etc.