"Sleep" spins down hard disks, darkens the monitor, does everything
possible to cut energy use short of turning the machine off. The
machine can recover from this state quite quickly.
Hibernation involves writing the contents of RAM and Video memory to a
disk file, then turning the machine off. No energy at all is used in
hibernation, since the machine is dead. When you boot your machine, it
looks for a hibernation file on the disk. If it finds one, then it
uses it to reinstate your previous session. If not, then it boots
normally.
Sleep mode status depends on how it is defined in BIOS.
E.g. in ACPI S1 - POS state ( power on standby )
all or most of components are on, just on low power state.
In ACPI S3 - STR state ( Save to RAM) all is switched off,
even CPU, only RAM is powered, slowly refreshed.
Hibernation is called also as ACPI S4 state - STD ( save to disk ).
Vista offers also so called hybrid mode, combining both.
it will save status to hibernation file in case of power failure,
bust stays is S1 or S3 power state, as Sleep does.