They are the same thing functionally, it's how it's done that makes the difference.
A reboot is actually powering down the computer, waiting 5-10 secs and then powering back on, thereby clearing everything out of the ram and starting all your processes back up. This is also known as a hard restart or a "not graceful" shutdown.
A restart on the other hand is initiated by windows, and conducts a graceful (or soft) shutdown where windows wirtes all information stored in the ram to the hard drive, sends stop commands to all of its processes and once everything is finished it cuts its own power and reboots right away.
The main difference is that with a soft shutdown, your information integrity is almost guaranteed. With a hard restart, your computer may have been in the middle of thinking about something and you could have possibly caused corruption of your drive etc. In a home system, this isn't too much of an issue, but on something like an exchange server if you do a hard restart you just corrupted your exchange databases (or any database running on that server really).
I've seen older winNT boxes take almost an hour to commit all the transaction logs into the information store and stop the exchange services before you could even think about touching the power button without information loss.......but that's another story I guess......