When you turn off your computer using hibernate, the operating system needs to store all your current running program states onto hard disk so that programs can resume where they left off when you power up your PC again. To achieve this, the OS need to save all the information into a file called hiberfil.sys. Windows OS (7, vista, XP) usually stores this as a hidden file under your C directory or your main OS partition.
The size of the hibernation file (hiberfil.sys) will be the same as the amount of installed RAM on your computer.So all the data from RAM is copied directly into this hiberfil.sys on harddisk and RAM loads it when you power up your computer again.
You can also use an Utility VJSTD made by VJ to automate hibernate & other shutdown options.