The Google Base data API is designed to enable developers to do two things:
Query Google Base data to create applications and mashups.
Input and manage Google Base items programmatically.
There are two item feeds: snippets feed and customer items feeds. The snippets feed contains all Google Base data and is available to anyone to query against without a need for authentication. The customer items feed is a customer-specific subset of data and only a customer/owner can access this feed to insert, update, or delete their own data. Queries are constructed the same way against both types of feeds.
See http://code.google.com/apis/base for more information about the Google Base API.