What is the difference between Sleep and Hibernate in Windows Vista?
Solution:
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