Designer Jonathan Ive was named as co-inventor on 64% of those 313.
Design patents cover non-functional ornamental aspects of the implementation. They are used to protect functionality-related aesthetics of the product: the look, the feel, the touch. Here's a couple of examples of Steve Jobs' patents:
(D638,835. Electronic device with graphical user interface -- looks like iPhone 4):
tags: patent, apple, innovation, creativity, interface