A mashup is a combination of data from two or more independent sources that is presented in an unanticipated or novel way. There are three main types of mashups: presentation layer mashups which re-present data visually; data mashups which combine datasets; and process/workflow mashups. The document discusses examples of mashups that combine APIs from Amazon and LibraryThing to present book data in new ways. It also describes how "glue logic" is used to connect incompatible data sources in a mashup.