Ever since the invention of APM (Advanced Power Management) in 1992, then ACPI (Advanced Configuration and Power Interface) in 1996, modern computers have had the ability to enter a low power mode typically referred to as "Sleep Mode", "Stand By", or "Suspend". Usually, this is done by cutting all power to the system, except that required to keep memory "alive".
You see this a lot with laptops, as it is a great way to save battery when you aren't going to use the laptop for a while. I used to do this all the time when I ran Linux on my laptop at work - worked great. If I needed to take my laptop with me to a meeting, I wouldn't shut down. I'd just put the laptop to sleep, bring it with me to the meeting - and if I needed the laptop, it took only a few seconds to resume.
But I probably used this feature the most when traveling. At a conference, I'd regularly check email during "down time". When it was time to attend a session, I'd put the computer to sleep, then during the next break I'd wake up the system and see if anyone had replied to my messages. It doesn't take long for Linux to do a hard boot, but it certainly took much less time to suspend the laptop, and resume later when I needed to use it.
Now that I'm forced to run Windows at work, I've tried to use the "Sleep" feature in the same way. I'll tell you, I'm not sure why Microsoft even bothers with this option. Under Windows Vista, there's a button that claims "Saves your session and puts the computer into a low-power state so you can quickly resume working." Technically, this is Hibernate, where the contents of RAM is written to non-volatile storage, such as the hard disk. Later, when you bring the system back up, the memory is read back from disk and things should be back where you left them.
However, I've never seen the point in how Vista goes into hibernation, and gets woken up again later. Things take forever to come back up. And if you changed anything while the computer was asleep, forget about it.
Example: Last night I was doing some work from home, finished, then put the computer to sleep. Never went back to it. This morning, I'm back in the office, put my laptop in the dock, and woke it up again. Everything was messed up, even to the point that basic USB devices like my keyboard and mouse weren't working. I ended up rebooting.
I pretty much just reboot Windows by default, rather than bother putting the laptop to sleep and waking it up again. It's too much bother to get Windows working again after things wake up. Really, it's not a useful feature if it takes me twice as long to get back to work by using Sleep Mode than if I'd just shut down the system and rebooted when I needed it.