Standby or the basic Sleep mode allows the computer to enter into a lower power consumption mode. The programs and data are stored in the memory. Supported by Windows operating system for quite a while now, the Sleep mode is different from the Hibernate feature (How to hibernate in windows XP) wherein your documents are saved onto the hard drive before the computer shuts down. Influenced by both these features, Vista has launched the Hybrid Sleep mode which is primarily aimed at desktop computers.
So how exactly is Sleep different from the Hybrid Sleep mode?
The power saving Sleep state allows the documents and other programs to be saved on the RAM while the rest of the system is powered off. Until the memory is powered, your existing data is safe and you can quickly resume full power operation. However if the power supply to the RAM is lost or the battery is discharged, you could end up losing the data. Moreover the computer has to be rebooted again. This problem has been resolved in Windows Vista installed on laptops and notebooks as the Sleep mode is enhanced to initially enter the standby mode and then when the battery is critically low, switch to the Hibernate function. To use the Sleep function, users need to click the Power button on the Start Menu or leave your computer inactive for some time.
Hybrid Sleep mode saves the documents and programs to the memory as well as the hard disk, while entering the Sleep mode. Thus in case of a power outage, your work can be restored from the hard drive where it is saved as hiberfil.sys hibernation file. The only drawback is that mode is slower than basic Sleep mode because it needs to save memory state into hibernation file. For using this mode, the function has to be enabled in Vista so that each time you click on Sleep in Power option, Vista will always put the computer into Hybrid Sleep mode.