Google Application APIs allow developers to write applications that access Google Applications such as Gmail, Calendar, and many others using Google Data APIs, Gadgets, and Google Apps Script.
Many other Google Application APIs are based on the Atom and RSS syndication formats and the Atom publishing protocol, and also include support for authentication, querying, and version conflict detection. Client libraries are available in a variety of popular languages.
To get started with Google Apps Application APIs, dive into the API-specific documentation. You'll find detailed reference documentation, how-to guides, discussion forums, and other helpful resources.