Having many apps running in the background is no big deal on mobile operating systems like Android. But, sometimes you just want to close or kill an erratically behaving app.
The easy method
Click the multi-tasking button. This is one of the three touch buttons at the bottom, typically the one on the right. It brings up a vertical list of open apps. Swipe the app you want to the right and it will be closed.
The next easier method
Open Settings > Apps, find your app and open it. A page with all the details of the app is displayed. Click the Force stop button here to close the app.
Tried with: CyanogenMod 10.1.2-encore and Nook Color