Closing an iPad app is usually just a matter of pressing the Home button to return to the iPad's Home screen. However, if an app is unresponsive, you can force it to close by pressing the Home button twice.
Force-Closing an App
Press the Home button twice to see the most recently-used apps, displayed in reverse-chronological order. Scroll left and right to find the app you want to close.
Drag the app preview up to the top of the screen to close it.
If an app freezes frequently, make sure you have the most recent version. Launch the App Store and tap the Updates button. Locate the app and tap Update. If a glitch is causing the app to freeze, there's a good chance that the update addresses that issue.
Does Force-Closing Improve Performance?
To aid in multitasking, the iPad keeps recently-used apps in standby-mode so that you can quickly access them without losing your place. The amount of memory that standby-mode uses is minimal, so in most cases you shouldn't have to force-close apps.
Some developers, however, recommend force-closing other apps as a troubleshooting measure when the app you're now using has become sluggish. This can save you from having to reboot or reset the iPad. This is usually the case only in resource-intensive apps like games with heavy graphics.