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.
Old Mac users like me were trained to quit any apps that weren't running. These old habits die hard, and at first it can feel odd that all of those apps are still "running." You can force them to quit, however. Here's how.
First, call up the Multitask Bar. Next, press-and-hold on an icon to enter "jiggle mode." Each icon presents a small, white "-" on a red field on its upper left-hand corner. Press it to remove that app from the list and terminate it (if it was running). Note that when you do this, you won't find that app in its previous state the next time it's launched. Why would you want to do this? I've found that it sometimes fixes misbehaving apps. Other than that, it's not necessary. As Steve Jobs said, just forget about it.
What's actually going on here?
Apps that "multitask" on the iOS are "suspended." GPS receivers, music players, etc. -- those that will operate in the background even as you perform another task in the foreground -- are running while suspended. Apple aims to keep as many apps running as memory will allow.
Otherwise, apps pick up where you left off the last time you used them. While waiting for you to return, those apps are suspended, living in a saved state for immediate return to the screen you were on when you switched to another app. If your iOS device runs out of memory, suspended apps that haven't been used in a while are purged from their suspended state. That decision isn't up to you, so you've got to trust Apple.
I'm loving this new feature, and hope you are, too. Now that I can hop from app to app without starting from scratch, my iPad is more useful than ever.