Support for Multiple Users on same device
This is available for phones starting in Android 5.0 (Lollipop):
http://www.android.com/versions/lollipop-5-0/

And for tablets starting in Android 4.2 (Jelly Bean):
http://developer.android.com/about/versions/jelly-bean.html#42-multiuser

As Tatjana Heuser mentioned, it's also available as hidden functionality in Android 4.1, but not accessible through the regular user interface.