The iPad (iPhone and iPod Touch) leaves apps in the background when you choose a new one. You can see these apps by double-taping the Home button. What you may not know is while most of these apps just sit there waiting for you to return to them, others are running and drain your battery a little faster than normal.
Watch this short, 2 minute and 43 second video demonstration.
You can close any or all of the apps by again, double tapping the Home button and then do a long press on one to get them to jiggle. Tap the red circle with the horizontal white line to stop the apps. Don't worry, the apps won't be deleted as this only occurs when the icons jiggle from the main screen.