IBM provides a very simple introductory article by Duane Merrill on web mashups which is good and easy reading for those new to the concept. It introduces the basic technical ideas and provides a solid set of linkages to further explore.
Mashups are certainly an exciting new genre of Web applications. The combination of data modeling technologies stemming from the Semantic Web domain and the maturation of loosely-coupled, service-oriented, platform-agnostic communication protocols is finally providing the infrastructure needed to start developing applications that can leverage and integrate the massive amount of information that is available on the Web. As mashup applications gain higher visibility, it will be interesting to see how the genre impacts social issues such as fair-use and intellectual property as well as other application domains that integrate data across organizational boundaries, such as grid computing and business-to-business workflow management.