What is an Apple ID?

An Apple ID is a combination of an email address and a password. You need this to perform certain Apple-related activities such as purchasing/downloading apps from the App Store and using the iCloud.

When creating an Apple ID (you only have to do this once), you need to enter your email address and choose a password. When using the Apple ID in practice, you mostly only need the password.

How to create an Apple ID, is explained in this article.