If you don't understand anything about API's then this is one Google tool you won't want to concern yourself with.
If however you're a programmer Google Data APIs Objective-C Client Library provides a Mac OS X framework and source code that make it easy to access data through Google Data APIs. GData is yet another simple but celever Google product that provides a protocol that allows you to not only access but also write data online. It is the protocol used by many of Google APIs, including Google Maps.
If you think about it, the number of resources this opens up is enormous including access data from various Google services like Google Base, Blogger, Calendar, Contacts, Picasa and even YouTube which you can freely export to your own applications. The GData website is a hugely comprehensive database of info, instructions and forums that will help even the most advanced programmers streamline their API coding.
GData is a typical Google product - simple, easy to use, powerful and completely free - could a programmer ask for more?.