from Msoft, from this article: http://windows.microsoft.com/en-us/windows-vista/Turn-off-a-computer-frequently-asked-questions
Sleep is a power-saving state. Sleep saves all open documents and programs, and allows the computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer to sleep is like pausing a DVD player: the computer immediately stops what it’s doing and is ready to start again when you want to resume working.
While your computer is asleep, its hardware lights may slowly flash or change color. This is normal.
You can read up on the different options in the article.