It's a programming tool to interact with Google, the services they offer and their databases. These are for advanced programmers using server side scripting. Have you done any API programming before? They can be awesome. Amazon offers API functionality too.
You can add some neat features just by using some of the demo code on Google site. (esp. the video search) .. I see a lot of sites feature APIs now .. amazon, youtube, google, yahoo, and a lot more . .