Web based maps are commonplace today. The catalyst for the explosive growth of web maps was the introduction of Google Maps. Web maps existed before, but they were not quick or developer friendly. In June 2005, Google released an API for Google Maps which provided a front end client (the role OpenLayers plays) along with an access to the backend map server.
This allowed anyone to insert not just a Google Map on their site, but also allowed them to add in their own point data and manipulate the map in other ways. Google Maps grew in popularity, and other companies, such as Microsoft and Yahoo!, followed in their footsteps, creating their own web-mapping APIs.
The term mashup refers to an application that combines various different data sources and functionality together. A map mashup is a map that combines different layers and data. Third party mapping APIs, like Google and Yahoo! Maps, allow people to more easily create these 'map mashups'. For example...