If you already use Google Checkout, the site will redirect you to wallet.google.com, where you can manage your transactions and payment methods. Google Checkout is now known as the Google Wallet Online Service, "a free service that lets you carry your wallet on the web". Not all the sellers will use the new name right away, so you'll still see Checkout's logo in many sites.
For now, the offline Google Wallet only works in the US and the Android app is only available if you buy a Sprint Nexus S 4G phone. "Our goal is to make it possible for you to add all of your payment cards to Google Wallet, so you can say goodbye to even the biggest traditional wallets. This is still just the beginning and while we're excited about this first step, we look forward to bringing Google Wallet to more phones in the future," said Google back in September, when Google Wallet was launched.