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See these links for information about Apple ID:
Frequently asked questions about Apple ID
Using your Apple ID for Apple services
An Apple ID is the email address you use as a login for just about everything you do with Apple, including using iCloud to store your content, buying songs from the iTunes Store, and downloading apps from the App Store.
See this article for some steps that can help if you're not able to log into iCloud with your Apple ID:
iCloud: Account troubleshooting