If you're new to the Google data APIs, here's how we recommend you get started:
The Google data APIs provide a simple standard protocol for reading and writing data on the web.
These APIs use either of two standard XML-based syndication formats: Atom or RSS. They also have a feed-publishing system that consists of the Atom publishing protocol plus some extensions (using Atom's standard extension model) for handling queries.
Many Google services support the Google data API protocol. The following data APIs allow your client applications to interact with Google services: