The email address you use to access many Apple resources, including iCloud services.
If you forget which Apple ID you use with iCloud, do any of the following:
On your iOS device with iOS 7 or later installed: Tap Settings > iCloud.
On your Mac with OS X Mavericks v10.9 or later installed: Choose Apple menu > System Preferences, then click iCloud.
On your Windows computer with the iCloud Control Panel: If you have Windows 8, move the pointer to the upper-right corner of the screen to show the Charms bar, click the Search charm, then click iCloud Control Panel on the left. If you have Windows 7 or Windows Vista, choose Start menu > All Programs >
iCloud >
iCloud.
For more information about your Apple ID, visit the Apple Support site.