Web Mashup = API [A] + API [B]
A web mashup is a website or web application that uses content from more than one source to create a completely new service.
Content used in mashups is typically sourced from a third party via a public interface or so called API.
Mashups, though they have been around since the first API was released, have begun to garner more media attention since the beginning of 2006 due to many web companies beginning to fully embrace Web 2.0 technologies that enable many people (and more specifically web developers) to easily interface with the primary company’s intellectual property.
A good example of this would be Google creating the Google Maps API and letting anyone utilize their Google Maps software and databases so that individuals can develop many new and creative uses for the mapping software.
Blog dedicated for Google Mapping Mashups
Source : WikiPedia