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iCloud Keychain: Password support
Among other things, iCloud Keychain allows you to forget the need to enter passwords -- at least on your trusted devices. Using the in-built browser Safari, Mavericks can generate random passwords that are entered automatically and stored in the cloud.
iCloud Keychain: Remembering card payment details
It works across devices, such as OS X machines and iPhones and iPads. In testing this, you can enter secure credit card data on a mobile phone top-up page and Safari will allow you to "remember" this card. The data will be stored in the cloud, allowing trusted Mac to use that data in future.
iCloud Keychain: Synchronizing card payment details in the cloud
And when you're back at your Mac and enter your credit card data that was stored by your mobile device, it grabs that data from iCloud.