In iOS6, you would double tap the home button, and all the running apps in the background would show along the bottom. Hold, they shake, and you would kill them. Not so in iOS7. How do we now kill the apps running in the background?
By the way, all the apps that appear there aren't really running in the background. Some might be, but vast majority aren't, they are just recent apps that were used, nothing more.
Double tap the home button, and swipe up on the app preview.
To those that are new to this, you can also swipe up 2 at a time.