On your iOS 4 and iOS 6 devices, you can quit apps by double clicking the home button, pressing on the icons in the fast app switcher tray until they jiggled and tapped the X to get rid of them.
If you have a lot of apps that you would like to discard from your iOS7, there is another way to do it.
- Launch the multitasking cards interface by double clicking on the home screen.
- Touch, hold a card, then toss it up and away.
- Touch and hold multiple cards to discard up to three apps.
Any app tossed off of the screen is shut down.