In the Apple world, the account that controls access to all your Apple-related online services is the Apple ID. Buying apps from the App Store, putting photos in iCloud Photos, and sharing data between iCloud-enabled apps—all these actions rely on your Apple ID. If you’re a regular Apple user, you have an Apple ID associated with your email address.
Most Apple users set up an Apple ID when they configure their first Apple device, and if you don’t have an email address that you want to use, you can create a free @icloud.com address during the process. (If you need to create a new Apple ID, you can do that at appleid.apple.com.)