Yes, it has taken me some time to decide to blog this, and even though I've seen some very nice examples, I've never payed too much attention to mashups. Interesting? Sure. Value? Unproven. What happens when the base code/API of the thing you've based all your work on changes? I have no idea.
A mashup is a website or web application that seamlessly combines content from more than one source into an integrated experience.
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 includes.
The etymology of this term almost certainly derives from its similar use in pop music.
Many people are experimenting with mashups using eBay, Amazon, Google, and Yahoos APIs.