A mashup is a website or web application that seamlessly combines content from more than one source into an integrated experience.
Much the way blogs revolutionized online publishing, mashups are revolutionizing web development by allowing anyone to combine existing data from sources like eBay, Amazon, Google, Windows Live and Yahoo in innovative ways. The greater availability of simple and lightweight API's have made mashups relatively easy to design. They require with 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."