search in this thread for "power save" and you will get the answers you seek.
what carrier are you on? I use sprint, and the power save icon is in the pull down drawer from the homescreen. i keep it off because i want the phone running for performance.
anyway, it has a little leaf in it and you can check it or uncheck it. when battery gets low, this "app" activities to help prolong battery life when the charge is on its last legs..