When should I turn off my computer and when should I put it to sleep?
Answer: Today's computers have super-efficient sleep modes that keep the state of your computer in memory, but power down all internal hardware. Unlike turning your computer off, open programs and windows are saved in their current state when your computer goes to sleep. This means when you "wake" the computer from sleep mode, you can pick up right where you left off. You don't have to go through the whole start-up process again.
Because sleep mode uses so little power, it makes sense to put the computer to sleep instead of turning it off each time your step away from your desk. However, if you are going to be gone for a few days, it may be best to turn the machine off. Personally, I put my computer to sleep every night and only turn it off if I am going to be gone for three days or more. I also have my computer set to go to sleep if it is inactive for 30 minutes.
The Sleep option in Mac OS X is located near the bottom of the Apple menu. Windows XP has Standby and Hibernate modes located in the Start menu. For more information on the difference between Standby and Hibernate, view this article.