How to use iCloud Keychain on iPhone and iPadWith iCloud Keychain, you can create and store logins and passwords, credit card details, and personal information across all your iPhone and iPad devices.
Updated April 2017: Rewrote to reflect the current iCloud and iCloud Keychain process.
These days, it seems like you need some kind of password or login information to visit practically every website. It is hard to keep track of all of those logins, not to mention recalling your credit card numbers and expiration dates when you are making purchases online at websites that don't have Apple Pay support. iCloud Keychain makes it possible for you to create and save all of your logins and passwords, as well as store your credit card details and personal information, like your home address and phone number, so you can quickly add them to fill-in forms without having to type the information out every time.
Consider a password manager, too
It is important to note that while iCloud Keychain is a fantastic program for storing your login...
Source:
How to use iCloud Keychain on iPhone and iPad