Multitasking on an iPad is more like old style task switching. Recent apps are frozen in RAM in their run state (with the possible exception of a few types of background tasks). You simple close one, and open the other to switch between them. When you go back to the first one, it should continue from where you left off.
To switch between open apps more quickly, you can double tap the home button. This will pull up the multi-task bar. This bar lists pretty much every app you've run, unless you've cleared it out recently. Only the most recent apps are actually in RAM. How many depends on your iPad. 3 to 5 for the iPad 1, more for the iPad 2. It depends on how big each app is.
You can close/remove apps from the multi-task bar. Tap and hold until they wiggle. Instead of the x to delete the apps like you would see on the app scree you will see a minus. Tap the minus and the app will be removed from RAM and the bar.
Another thread that should be more helpful. It's got a video that's worth a lot more words than I'm likely to type here.
http://www.ipadforums.net/apple-ipad-news/14143-ipad-multitasking-4-2-explained.html