See What Apps Are Currently Running on Android

Android apps often keep running even after you switch away from them. It's useful to look up exactly which apps are still running as a result.

Steps

Go to the Settings menu on your phone. Then, select More and then Application Manager.
Pull the title bar in apps over from the right. Stop when you see "running."
Click on the "running" option.
Look at the apps you currently have running. This includes the memory used by each listed on the right.

An app like Task Killer can be a good way to shut down apps that you don't want running.