The Multitask Bar
The iOS keeps a running tally of recently-used apps for you and displays them in the Multitask Bar. To bring it up, just double-tap the iPad's home button as you do on the iPhone. The main screen slides out of the way to reveal the list. There are two main differences between the Multitask Bar on the iPad and the iPhone.
First, the iPad lists the six most recently-used apps while in portrait orientation and seven while in landscape. The iPhone always lists four, regardless of orientation. Also, swipe all the way to the right on the iPhone to bring up the orientation lock button, iPod controls (back, play/pause and skip) plus the iPod app icon. The iPad displays those, too, as well as a brightness slider and a volume slider.
This is minor but still fun. The animation that the iPad uses when fast-switching between apps is slightly different than what the iPhone uses.