An activity that Windows users partake in more often than playing Solitaire. Done either when the computer freezes (thus making a soft reboot impossible), or when the computer is being malignant enough to make a hard reboot feel good.
It is done by either pressing the power button (on newer systems, it is required to hold the button) or unplugging the computer from the surge protector (you do use a surge protector, right?). Often frowned upon, as it can (though has not in my experience) wreck various things on your system. One side-effect on a Windows machine is that when the computer is booted back up again, Scan Disk is run unless turned off by Tweak UI or some other manner, and will increase the boot time greatly unless you catch it and press 'X'.
A hard reboot is usually done in the hopes that once the computer is booted up again, it will be working better than it was before.