Here's the key difference for Windows 8: as in Windows 7, Windows 8 closes the user sessions, but instead of closing the kernel session, it comes to a hibernation state. Compared to a full hibernate, which includes a lot of memory pages in use by apps, session 0 hibernation data is much smaller, which takes substantially less time to write to disk. For those not familiar with hibernation, the OS is effectively saving the system state and memory contents to a file on disk (hiberfil.sys) and then reading that back in on resume and restoring contents back to memory.
Using this technique with boot gives Windows 8 a significant advantage for boot times, since reading the hiberfile in and reinitializing drivers is much faster on most systems (30-70% faster than Windows 7 on most systems according to Microsoft). It's faster because resuming the hibernated system session is comparatively less work than doing a full system initialization, but it's also faster because Microsoft added a new multi-phase resume capability, which is able to use all of the cores in a multi-core system in parallel, to split the work of reading from the hiberfile and decompressing the contents. For those users who prefer hibernating, this also results in faster resumes from hibernate as well.
Another important thing to note about Windows 8's fast startup mode is that, while Windows 8 does not do a full "Plug & Play" enumeration of all drivers, it still initializes drivers in this mode. Microsoft says that this new fast startup mode will yield benefits on almost all systems, whether they have a spinning HDD or a solid state drive (SSD).
In addition, Windows 8's POST handoff occures very fast. Systems that are built using Unified Extensible Firmware Interface (UEFI) are more likely to achieve very fast pre-boot times when compared to those with traditional BIOS. This isn?t because UEFI is inherently faster, but because UEFI writers starting from scratch are more able to optimize their implementation rather than building upon a BIOS implementation that may be many years old.
Of course, there are times where users may want to perform a complete shutdown ? for example, if they're opening the system to add or change some hardware. Windows 8 will have an option in the UI to revert back to the Windows 7 shutdown/cold boot behavior, or since that's likely a fairly infrequent thing, they can use the new /full switch on shutdown.exe. From a cmd prompt, run: shutdown /s /full / t 0 to invoke an immediate full shutdown. Also, choosing Restart from the UI will do a full shutdown, followed by a cold boot.