Sleep on a Mac is similar to Sleep on a windows pc (although, I've had sleep on my Mac work correctly and have always had trouble with it on my pcs) - if you loose power in sleep mode, the machine will need to do a full reboot, unlike Windows Hibernate which will resume from the HD basically by using a saved copy of what's in RAM.
Now, a little search on google for OS X Hibernate did yield an interesting piece of info about "safe sleep". It looks like it doesn't work necessarily for everyone, but it for those that it does work for - it's similar to Hibernate from what I've read..
Mac Life. Simple Life.: Mac OS X Hibernate Issue