What is the difference between Sleep and Hibernate in Windows Vista?
Sleep is commonly known as Standby in Windows systems or S3 in ACPI. In Sleep mode, the power supply to non-essential and non-critical components is withheld, and most system operations are shutdown and stopped. All data in physical memory (RAM) is still kept in internal memory, and the whole system is placed in a standby mode, which can be woken up and used almost immediately.
In Sleep mode, the power load reduces considerably, saving energy. However, the power to the machine must not be cut off completely. If power is lost, the system state will also be lost and the computer will behave as though just booted from a powered-off state.
Hibernate, or S4 in ACPI, meanwhile will save the data in physical memory to the hard disk drive first, and then power off the computer.
In Hibernate mode, a file named hiberfil.sys, which has the same file size as the amount of system memory, will be created on the local disk. When the user restarts the machine, it will boot up and load back the system state at the point of the last hibernation.
The advantage of Hibernation mode is that no power is wasted. In Hibernation, no electricity is consumed by the system so a computer can be kept in this state indefinitely.
Also, restoring from Hibernate is generally faster than a computer reboot. It is also different from a reboot in the sense that users can return to the exact state of the last hibernation with all running programs and open documents intact, instead of booting to an empty desktop.
The disadvantage of Hibernation is that after a period of time, there may be fragmentation of the hiberation file. Users may need to defragment the volume that stores the hibernation file frequently.
Answer adapted from here
In short: Sleep takes power, hibernate doesn’t
Sleep still keeps your RAM (working memory) active, so the start-up time is faster but it takes a little power to keep the computer in sleep mode.
Hibernate, copies the RAM, to the hard disk. This doesn’t need power to keep going, but when you start up, the RAM needs to be copied from the hard disk to the RAM, which takes time.
Sleep is useful when you want to be able to start up instantly, and you won't be without power (if you're using a laptop) for less than a day. If you want to be able to start up quickly (faster than normal boot, but slower than sleep), you can use Hibernate, which allows you to put your computer to sleep for days without losing your data.
"Sleep is a power-saving state. Sleep saves all open documents and programs, and allows the computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer to sleep is like pausing a DVD player: the computer immediately stops what it’s doing and is ready to start again when you want to resume working."
"Like sleep, hibernation is a power-saving state. But while sleep typically saves your settings in memory and draws a small amount of power, hibernation saves your open documents and programs to your hard disk and then turns off your computer. Of all the power-saving states in Windows, hibernation uses the least amount of power."
from Windows Vista Help