Advanced Configuration and Power Interface -
is an industry specification developedby giants such as HP, Intel, Microsoft, Phoenix, and Toshiba.
ACPI suggests standard interfaces for power management on computers, wether for personal use or high-end servers.
ACPI builds on the legacy power management BIOS code, the previous Advanced Power Management (APM) APIs, PNPBIOS APIs, and even Multiprocessor Specification (MPS) tables in a way to provide a specification for power management able to work on multiple hardware and software platforms.
While it is partiually implemented on both firmware and software end points, as the other W/U states, it brings no more than headaches and pretty buzz-words for developers and computer users alike.
Info picked up from:
http://www.acpi.info