GNOME Power Manager is a session daemon that takes care of power management. GNOME Power Manager uses information provided by HAL to display icons and handle system and user actions in a GNOME session. Authorised users can set policy and change preferences.
GNOME Power Manager acts as a policy agent on top of the Project Utopia stack, which includes the kernel, hotplug, udev, and HAL. GNOME Power Manager listens for HAL events and responds with user-configurable reactions.
The main focus here is the user interface; e.g. allowing configuration of power management from the desktop in a sane way (no need for root password, and no need to edit configuration files)
Most of the backend code is actually in HAL for abstracting various power aware devices (UPS's) and frameworks (ACPI, PMU, APM etc.) - so the desktop parts are fairly lightweight and straightforward.