ACPI stands for Advanced Configuration and Power Interface. This is a specification by Toshiba, Intel and Microsoft. Besides many other things it also defines power management. This is why it is often compared to APM.

You might use the following parameters (with the appropriate changes) in your boot loader configuration file (e.g. /etc/lilo.conf to experiment with ACPI and APM, when compiled in the same kernel. Usage of APM and ACPI at the same time doesn't work, see Kernel docs for details.

append="acpi=on apm=off"

The Linux ACPI Project is committed to the development of fundamental ACPI (Advanced Configuration and Power Interface) components for Linux. This includes a generic ACPI table parser, AML interpreter, bus and device drivers, policy, user interface, and support tools.

The E-AcpiPower epplet is based on E-Power. It is modified to read battery status information using the new acpi kernel module, making it much more accurate and reliable than the old APM method.

TCL/TK script which allows setting the ACPI CPU performance state using a graphical interface under Linux.

Linux ACPI client is a command-line tool, similar to the apm command, that provides information on battery status, AC power, and thermal readings.