In Windows Vista, 'Stand By' and 'Hibernate' have been combined into an additional 'Sleep' function which is active by default. When chosen, this new 'Sleep" mode saves information from the computer's memory to the hibernation file on disk, but instead of turning off the computer, it simultaneously enters Standby mode. After a specified amount of time (3 hours by default), it shuts down (hibernates). If power is lost during Standby mode, the system resumes from the existing hibernate image on disk. Sleep mode, thus, offers the benefits of fast suspend and resume when in Standby mode and reliability when resuming from hibernation, in case of power loss. Also, in earlier Windows versions, drivers sometimes prevented Windows from entering or reliably resuming from a power-saving state. Windows Vista ensures the availability and reliable resuming from any power state. Applications can disable sleep idle timers when needed such as when burning discs or recording media. Away mode, which is not a power plan by itself but a feature, automatically turns off displays, video rendering and sound but keeps the computer working when the user is away from the computer. Optionally, it can also transition to sleep mode. Power settings are also configurable through Group Policy.
Hibernate takes a snapshot of everything you got on RAM (including any windows and apps running) and saves it to a special hard disk file and then shuts the computer down, when you resume from hibernation the computer boots a bit faster than a normal power up bootup. This method does not consume any power while the computer is off.
Uses: When you are not going to use the computer for a few hours or more.
Sleep, basically shuts your computer down except some components like RAM, so the next time you resume, it will boot almost in an instant. However, this method consumes battery power.
Uses: When you are just leaving the computer for a few minutes or hours, like at lunchtime or something like that.
