Windows 8 Fast Startup
In Win8 RTM version, this feature is called "Fast Startup"; In the previous versions of Win8, it was called "Hybrid Boot"; In Win7, it was called "Hybrid Sleep".
The idea is to keep your computer sleep when it is not being used. The memory are kept in RAM with small amount of electricity expense. Then it can be waken up quickly, to give user the best experience. When you turn on TV, you don't see a progress bar saying "Starting up...20%...24%...25%..." With this new technology, Microsoft brings up the computer as fast as possible.
This new feature is different from the old "sleep" because it can go into deep sleep, hibernate, when the power is lower than a specific lever. Assume you forgot to connect power cord to the poor laptop during weekend, it can save all data from RAM into harddrive, then turn off itself completely, when it detects danger. That is why it was called "hybrid": It is a combination of sleep and hibernation.
The new thing about this feature is that it fake itself as "power off". When you press power button (the physical power button of your computer), it goes into this hybrid sleep mode immediately. This works well for laptop or netbook or any electronic device, because they have batteries. But for traditional desktop, this is not ideal: The user might think it is powered off, so he unplug the computer to move it to another place. Then the data in RAM is wiped out.