S0 (Working) is lit. The CPU functions in entirety; the conservation of power is on a basis of by-device.
S1 (Sleep). The CPU is stopped; The RAM is regenerated; the system functions in reduced power.
S2 (Sleep 2) The CPU does not have any power; The RAM is regenerated; the system is in lower mode of the S1.
S3 (Standby) The CPU does not have any power; The RAM regenerates at minimum; the power supply unit is in mode of reduced power. This mode is also called “Save to RAM”.
S4 (Hibernate). All is powered off, but the memory was saved like temporary file on the hard drive. This mode is called “Save to disk”.
S5 (Off) Everything is shutdown. Means you need a full reboot.
S3 is the best. It uses less power and reboots faster.
Vista and Win 7 have hybrid sleep.
So what is Hybrid Sleep?
Hybrid Sleep is a low-power sleep state that can also protect your open programs and documents in current working state as sysem enter Sleep state while your computer sleeps. Hybrid sleep is defined mostly for desktop system as notebook PC already has safeguard measure mentioned above, which brings system into hibernation when battery power almost runs out, or if you’re not using battery on your notebook computer.
In Hybrid Sleep mode, system saves any open documents and programs to memory and to your hard disk, and then puts your computer into a low-power state as in normal Sleep/Standby state. An advantage of hybrid sleep is that if a power failure occurs, Windows can restore your work from your hard disk. If a power failure such as power outage occurs when your work is saved only to memory (as in Sleep mode), all work is lost. In Hyrid Sleep mode, if the computer suddenly loses power, users can still recover to last working state as when computer enters hybrid sleep mode because all data in memory is saved in hiberfil.sys hibernation file. In this case, the computer is acted exactly same with hibernation mode to resume computer activity, and yet still enjoy the benefit of fast return to full operation of Sleep mode.
However, Hybrid Sleep mode takes longer or slower than basic Sleep mode due to the fact that it will need to save memory state into hibernation file as it was going into hibernate before actually going into sleep. You won’t see Hybrid Sleep button in Vista though. Once you enable Hybrid Sleep mode, whenever system enter Sleep mode, i.e. click on Sleep in Power option, Vista will always put the computer into Hybrid Sleep mode. If the Hybrid Sleep support is disabled or turned off, Sleep button will continue to be normal Sleep button.