Your Apple ID is used to log into iTunes to download iPhone or iPad apps to your device.
If someone else has preconfigured your iPhone it may still have their Apple ID set up. Or if you have ever wondered why your App downloads appear on your partners device it could be your Apple ID is loaded on their device!
To check whose Apple ID has been logged into your device, or to change to a different Apple ID:
Your Apple ID will be listed.
To sign out:
Tape your Apple ID
Tap Sign Out