Justintime is right. If you need to restart, do it from the Start menu if possible. Sometimes the PC will freeze, and the only way is to hit the restart button. And what Justintime just mentioned, it will restart without saving your work.
Nice definition glussier. Of course, the reason you use the reset button rather than turning the power off and on again is because the power surge caused by powering up and down (particularly in rapid succession) is bad for many of the components and could shorten their life.
That's why some people leave their systems on 24/7, although there's been plenty of arguments on HWC over the years about the merits of that strategy.
a hard reboot - power off/on or using the reset switch is the same as turning on the machine for the first time - all devices are reset and powered up/initialised completely blanking any residual setting
a soft reboot - ctrl,alt,delete restarts the computer from the moment past initialising the hardware which is why the memory isn't retested at startup. This i believe can leave residual settings in hardware/memory etc and potentially virus's so is fine for resetting machine if possible after a common or garden software crash but if you've used software to change hardwaresystem settings e.g. agp fast write(prehaps?) you need a hard reboot or obviously if the system is no longer responding to ctrl alt del.
think thats right, or least ways right enough for the average layman!