Originally Posted by DDDa
@YinYang: It's very simple. When in sleep, the computer goes to a state that it won't use too much power (very little), but the data is still there (volatile). So, if you turn the computer off (or take the battery out, for example), the data is lost...
To enable hibernation, you need to activate it as GOLD-BEATER said, but to use it, you go to power off and that blue window with "sleep - reboot - turn off" will appear. If you press shift, you will see that the sleep option will change to hibernate...
edit: what makes resuming hibernation faster than regular boot (the more start up process and boot time routines you have, bigger this difference will be) is because it just makes kind of a 1:1 copy (not 100% sure, but I believe this is more or less how it works) of the hibernate section on the hard disk, instead of processing all the routines and programs to bring the desktop OS from scratch to a "ready to work" mode.
You are 100% correct about Hibernate making a RAM image on the HD, making it 100% non-volatile and consuming zero power when sleeping. And yes it is lots quicker to use already established parameters (Hibernate) vs re-starting and setting parameters for all your programs and services (Re-Boot).