If you put your computer into hibernation, the next time you turn it on, you'll get all your windows and programs open just the way they were before the shutdown.
If on your Windows 7 computer opening programs, saving documents, browsing the Internet and other every day tasks are getting slower, this is how you speed up things without installing an additional program.
In this day of age, it's very practical to have a Windows 7 installation on your USB key, for many netbooks and devices don't come with CD/DVD drives anymore.
