You invoke the Multitasking screen on your iOS device by double clicking on the Home button. The card-based interface allows you to quickly browse through all your background apps. Tap on the app screen or app icon to go to the app; or you can quit the app by just flicking the app screen up and off the screen.
There is no way to close all background apps at once. But you can make use of multitouch support on the Multitasking screen to make the job faster. On the multitasking screen, select multiple apps with your fingers and flick the app screens up and off the screen at once.