MS Windows has 2 "sleep modes" , these modes let you keep the state of the computer (running programs, open documents etc) active, while being away from the computer for extended periods of time. The main difference between both is that in "Standby" all you open programs  and documents are stored on RAM. This means that coming back up from "Standby" is almost instantaneous (because of the speed of the RAM) but RAM needs electricity to store information, so your computer is consuming power while in "Standby". "Hibernation" on the other hand stores your computer state to your hard drive and consumes no power, however it takes much longer to come back up than "Standby". So it is a choice between power consumption, versus loading speed. I use "Hibernate" in my laptop and "Satndby" in my desktops.

Hope this helps