The ACPI sleep states S0 to S5 are familiar to many users. However, the ACPI specification actually describes several different power states. One of these is the CPU (C) power states.
ACPI allows processors to enter a reduced power state – to effectively sleep – and modern hardware makes this possible. The CPU power states apply when the global state is G0. The C states are:
C0 – the CPU is actively executing instructions;
C1 – the CPU is not actively executing instructions, however it can resume executing instructions almost instantly; and
C2 and C3 – the CPU is in a reduced power state; no instructions are being executed and the processor consumes less power; C2 and C3 are sometimes called Deep Sleep, because they represent progressively deeper levels of power-saving compared to the more shallow sleep states like C1.
ACPI allows vendors to define additional power saving states beyond C3. Since these are vendor-specific, you'll have to ask the manufacturer of your computer to explain them,