Hybrid sleep is a new power saving state for desktops running Windows Vista. Hybrid sleep is designed to give users a fast on and off experience similar to an appliance yet save energy. It accomplishes this by saving user documents and programs to memory and also to your hard disk then sending the computer to a low power state.
A common question comes from confusing Hybrid sleep with Away Mode as both are similar in appearance but Away Mode utilizes more power as it is not entering a low power state. Away Mode disables the audio and video from the PC giving the apearance that it is off. It's similar to standby on an appliance.
Away Mode is the setting I use on my Windows Vista Media Center. If you want to save more energy and your machine supports Hybrid sleep, enabling Hybrid sleep may be a great alternative to Away Mode. You can even determine the difference in power usage with a wattage meter to determine what your energy savings are compared to just leaving your computer on or using Away Mode. Hybrid also offers a little more protection from sudden power failures than Away Mode. As a small hibernation file is written to the disk so if the power goes out suddenly, you will have a greater chance of recovering the data. Also note that enabling Hybrid sleep will disable Hibernation from showing up in the Windows Shutdown menu.
To enable Hybrid Sleep: