Stand By is just like Stand By on your tv, it just puts the computer into a state where the computer is barely using any power (my computer turns off, but when I press the power button, it loads straight to where I set it to stand by from and doesn't go through the POST proceedure)
You can leave your computer in stand-by as long as you want.... Some people don't even turn their computers off, they just put them in stand by
It uses, on average, around half of the power the computer usually uses....
Hope that helps