ZDNet Executive Editor (and overall nice guy) David Berlind, typical of his style, gives an energetic, visual, and clear guide through the world of mashups for those who are both non-technically inclined, and tech savvy non-geeks.
A mashup is when a developer takes API's from multiple web sites and starts merging them, or 'mashing' them together in a way that forms a new, cool, innovative application that was never before on the web.
Mashups are the fastest-growing application ecosystem in the world today. It's going to only get faster. All of these different mashups are showing up right now at the rate of about 2.5 a day. Over time, this is going to go up to about 10 mashups a day -- probably within the next year. Pretty soon, the growth of this ecosystem is going to outstrip the growth of any previous operating system-based ecosystem that ever existed before.