You can easily swap between apps without having to close the one you’re using. Double-press the Home button quickly to bring up the multitasking view. This reveals your apps in the order they were opened as well as a screenshot of what you were doing.
Swipe from right to left to see more and tap on an icon or screenshot to switch back to that app.
To close an open app, swipe up on the screenshot. Close multiple apps at a time by using two or three fingers – simple swipe up using the same motion to close them.