The BIOS power management state

S0
The system is turned on. The CPU is executing or ready to execute instructions, PCI activity is full, AGP activity is full. RAM is being read from, written to or refreshed. Hard disks are on.

S1 “Power On Suspend/Stopgrant” MS: Standby
The system is turned on. The CPU is not executing and is not ready to execute instructions, although registers and caches are maintained. Devices signalling support for S1 are in the on state, devices without support for S1 are in the off state. RAM is idle, but refreshed. Any device currently in S1 with support for resuming may resume the system (WOL, WOR, keyboard, mouse, timer, etc.). PSU state is on. Hard disks are off in this state and all states deeper.

S2 “Standby”
Confusing “common” names. S2 is quite logically an intermediate between S1 and S3. RAM refreshes normally. The CPU is in state much like S3. In fact, this is pretty much S3 but with a faster RAM refresh.
S2 is in the spec, but not usually implemented.
S3 This BIOS management state shuts down all components, including the CPU, but the RAM remains powered on. The system is able to respond to wake up commands, such as a key stroke on the keyboard. This state can be referred to as standby or suspend mode

S4 “Suspend to disk” MS: Hibernate
All hardware is in the off state and maintains no context. The system may only be resumed by timer or other hardware resume devices (such as WOL or WOR), but not by interrupts. The contents of RAM are saved to disk and replaced on resume. The PSU is in the off state. Power may be mechanically removed without ill effect.
Note: S4BIOS is a variation of S4 wherein the BIOS reloads the image and not the OS boot loader. As NTLDR is S4 compatible, this is generally used with other OS’. Linux S4BIOS support is “experimental”. Note also that using Windows2000 or XP with grub or lilo WILL break S4 support.

S5 “Soft-Off” MS: Hibernate
All hardware is in the off state and maintains no context. CMOS is maintained by 5Vsb, as in S4. The PSU is in off state. Power may be mechanically removed without ill effect.