In the world of power management ACPI is relatively new to the game. It was first released in 1996 by Compaq/Hewlett-Packard, Intel, Microsoft, Phoenix and Toshiba. These developers aimed to replace the previous industry standard for power management. Their ACPI.info site contains the official specifications, a list of companies that support ACPI and a number of other goodies. This is definitely not required reading, but may be of some interest to the insanely curious.
ACPI allows control of power management from within the operating system. The previous industry standard for power management, Advanced Power Management (APM), is controlled at the BIOS level. APM is activated when the system becomes idle--the longer the system idles, the less power it consumes (e.g. screen saver vs. sleep vs. suspend). In APM, the operating system has no knowledge of when the system will change power states.
ACPI can typically be configured from within the operating system. This is unlike APM where configuration often involves rebooting and entering the BIOS configuration screens to set parameters.
ACPI has several different software components:
a subsystem which controls hardware states and functions that may have previously been in the BIOS configuration
These states include:
power states (sleep, suspend)
a policy manager, which is software that sits on top of the operating system and allows user input on the system policies
the ACPI also has device drivers that control/monitor devices such as a laptop battery, SMBus (communication/transmission path) and EC (embedded controller).
If you would like more information on power management in laptops, check out the resources on tuxmobil.org. Specifically: Power Management with Linux - APM, ACPI, PMU and the Hardware in Detail section of the Linux Mobile Guide.