Most mashups cash in with Google’s advertising engine on their sites, splitting the take with Google. If you have just one page, you don’t make much, but some have many more. (Yahoo! has a similar service, called publisher’s network, which is less broadly used.) Google also wants to grow the Internet as much as possible–the more interesting things there are, the more people will need Google to find them. And the better it understands what users are doing with a mashup–something that only the owners of the original data see best–the better its search results.
The term “mashup” comes from music, when two or more songs are mixed together to build a new sound. In software, they arise when someone develops a company’s online maps, calendars, photos, search engines and other products, usually through relatively small amounts of code that is posted on the Internet. These outside developers take the application to new uses, usually through combining the data of several sites.