Google checkout enables us to process customer orders in two ways,

Option 1: Integrate only your shopping cart (easier: requires only a working knowledge of HTML)

• Send shopping carts to Google using HTML code you add to your site.
• Process orders through the Google Checkout Merchant Center.
• Integrate using the HTML API Developer's Guide

Option 2: Integrate your cart AND order processing system (more complex: requires API programming experience)

• Send shopping carts to Google via an XML-based API.
• Process orders through your existing order processing system.
• Integrate using the XML API Developer's Guide

In option 1, we have to build one form in html with hidden fields containing necessary information for Google Checkout process. Orders will then be processed through Google Checkout Merchant Account.

While in Option 2, we can integrate our existing order processing system with the Google checkout with XML API.

In Option 1, customer will have to log in into his customer Account for Google checkout, and then he can view his purchase information which is sent by us (Our site) with hidden fields, in HTML form.

Same way, we can (Site Administrator) analyze the order information and can do operations like charging the customer for his order.

Getting Started Doc for Google Checkout is,

Google Checkout