From: https://discussions.apple.com/thread/3097140?tstart=0

<copy> Retired Engineer Jun 4, 2011 4:25 AM in response to julyjade24 http://support.apple.com/kb/HT1757

Deep Sleep is the equivalent of Windows hibernation. The state of the machine is written to the hard disk and the power is turned off.

Safe Sleep is the first part of Deep Sleep. Before the MBP goes into regular sleep the state of the machine is written to the hard disk. That's why it takes a while for the indicator to begin pulsing. If the battery dies while in regular sleep nothing is lost because of the write to the hard disk. Regular sleep becomes Deep sleep if the battery dies. </copy>

From: https://msdn.microsoft.com/en-us/library/windows/hardware/ff564575(v=vs.85).aspx

System Sleeping States

System Power State S4
System power state S4, the hibernate state, is the lowest-powered sleeping state and has the longest 
wake-up latency. To reduce power consumption to a minimum, the hardware powers off all devices. 
Operating system context, however, is maintained in a hibernate file (an image of memory) that the 
system writes to disk before entering the S4 state. Upon restart, the loader reads this file and 
jumps to the system's previous, prehibernation location.
If a computer in state S1, S2, or S3 loses all AC or battery power, it loses system hardware context 
and therefore must reboot to return to S0. A computer in state S4, however, can restart from its 
previous location even after it loses battery or AC power because operating system context is retained 
in the hibernate file. A computer in the hibernate state uses no power (with the possible exception of 
trickle current).