This is straight from the Android Lollipop features website:
Device Sharing:
More flexible sharing with family and friends
Multiple users for phones. If you forget your phone, you still can call any of your friends (or access any of your messages, photos etc.) by simply logging into another Android phone running Lollipop. Also perfect for families who want to share a phone, but not their stuff.
Guest user for phones and tablets means you can lend your device and not your info.
Screen pinning: pin your screen so another user can access just that content without messing with your other stuff.