This is a specific type of US patent filing. It relates to the underlying form and style of the device. Apple said that a 'white rectangle' with 'large borders/margins' and 'curved corners' was their 'signature' design features for a 'flat tablet' computer device. The jury agreed (very strongly).
I personally don't agree this type of patent should be used so broadly, or provide protection that strong. That said, the Apple court argument does have some merit.
The British / Common Law approach, providing protection against "Passing Off", is (IMHO) a much better approach. This protects anyone (even without specific 'design' patent filing) from a product (particular an inferior one) from being too close in exact appearance.
I hope (in the Apple / Samsung case) the jury finding is overturned or weakened on appeal, to be closer to "Passing Off" protection, and simply requiring Samsung to diverge enough in their design.