From their site: Google Checkout is a checkout process that you integrate with your website, enabling your customers to buy from you quickly and securely, using a single username and password. Once they do, you can use Google Checkout to charge their credit cards, process their orders, and receive payment in your bank account.
Click the "Track this API" button on any profile page and never miss an API update, new app, or breaking news for that API again.
It is useful because PayPal is not available in many countries.
Google checkout is great but still paypal has more advantage like they are accepting payment from 55 countries and google only from two.