ACPI (Advanced Configuration Power Interface) refers to a Power Management specification developed by Dell, HP, Intel, Microsoft, Phoenix, Toshiba, and Dell.
The ACPI specification allows an operating system to control the power distribution to peripherals, such as hard drives. It was originally intended for laptops/notebooks where power saving is vital.
Since Windows 98, ACPI has replaced the older APM (Advanced Power Management) specification (used in Windows 95).
ACPI allows the operating system to communicate with the computer's BIOS and instruct the BIOS to power down peripherals. For instance, when your computer goes into hibernation mode, the operating system is using the ACPI specification to control the power to the internal components.