How to kill running application?

Hello
I am just wondering how could I close application which I am not interested any more after I launched it. I understand that if I switch to another one it will run in background consuming some space. Not sure how it'll impact on device performance but I just think that less app using memory it should perform better.
Well hoe could I kill that running app?
Thanks

 

Double tap the home button, hold down the app until it jiggles then click the red circle.

 

Great!
Thanks

 

Doing this frequently should help save your battery some as well!

 

If you have gestures turned on, you can do a four finger swipe up to see the open apps. Do the four finger swipe down to close the bar.