When I double click my IPAD screen there are apps at the bottom. How can I remove them?

If you have iOS 7, there is material just above each app's icon...swipe that material up and off the screen and the app will be dismissed.

If you are running iOS 6 or iOS 5, those apps are the recently used apps in the recents tray. You don't need to remove then from there, but if you want to do so, you have to close them. You do that by holding down on one of the app icons until they all wiggle, then tap the red minus sign to close the app. Do that to every app that you want to close. Tap the home button when you are done.