BIOS is where you can: check the settings of your CPU/mobo, you can also change voltages, change sleep states (hibernation and sleep), choose what your primary video output comes from (PCI/PCI-E/Integrated), manage RAID, blah blah blah. Depending on your mobo and it's BIOS version, you can manage many things in the BIOS.
ACPI is a power-saving feature on Intel systems with Windows, it allows you to turn off/use low-power for USB or other peripheral device. A neat thing is that manufacturers can use ACPI to power up a computer with just a touch of a keyboard or mouse.
CMOS stores your BIOS' memory of the date/time and settings.