In iOS 6 and 7 there's a section in the Photos app that allows you to share photos with other people and other people to share photos with you.
- When others share with me, are the photos copied to my harddrive? Or do they purely reside in the cloud?
- If I add the same photo to two different streams, will it duplicate the amount of space used on my phone? Or does Apple intelligently make a reference link instead of a copy?