Google's mission is to organize the world's information and make it universally accessible and useful. Sometimes making information accessible requires making it available in contexts other than a web browser. Thus, Google provides APIs to let client software request information outside of a browser context.
GData provides a general model for feeds, queries, and results. You can use it to send queries and updates to any service that has a GData interface.
Syndication is an effective and popular method for providing and aggregating content. GData provides a way to expand the types of content that Google can make available through syndication; in particular, it lets you use the syndication mechanism to send queries and receive query results. GData also lets you send data to Google, and update data that Google already has.
