Hiberfil.sys is a file that stores everything currently in your system memory so that should you hibernate your PC, it will be able to shut down everything and then fairly quickly boot back up and reload all the data back into your memory. The difference between hibernate and sleep is that sleep will actually just keep your data in the memory, meaning the PC will still draw some power to do so (if the RAM loses power, it loses the data).
But do you ever actually use hibernate? This is a feature that is really meant to help boot the PC up quicker, much like sleep, but at the same time it's supposed to save a little more power. Personally, I just shut my PC off unless I'm going to use it again soon, in which case sleep mode is just fine.
So why do I still have this gigantic hiberfil.sys file eating up my SSD space? By default, hiberfil.sys is 75% of the size of your total RAM, so in my case that's 6gb of space uselessly occupied on my 80gb SSD.
To get rid of it, at least in Windows 7, is pretty easy. Simply open a command prompt as Administrator (Start button -> type "cmd" in the search bar -> Right click on Cmd and then Run as Administrator) and type:
powercfg -h off
Done. This will disable hibernate as well as delete the hiberfil.sys file.