Greenify help you identify and put the misbehaving apps into hibernation when you are not using them, to stop them from lagging your device and leeching the battery, in an unique way! They can do nothing without explicit launch by you or other apps, while still preserving full functionality when running in foreground, similar to iOS apps!
NEW: Non-root working mode is now supported in 2.0, but still in early stage. It currently lacks auto-hibernation due to privilege restriction. Use "Hibernate All" shortcut (widget) for convenience in non-root mode.