a few answers for your multitude of questions:
a tire iron is what you use to pop the tire off the rim when you get a flat - it's a little plastic lever with a lip on one side to wedge under the tire bead.
A tube would be an inter tube - you can keep one in your seat pack along with your tire levers and pumps so that when you get a flat you can replace the tube, rather than patching the old one.
You are allowed to ride a road bike on gravel - it's called cycle-cross, but be prepared for pretty rough handling and more wear and tear on your new bike.
As far as little debris in the road - you will feel it a lot more, and with skinny tires it can set you off balance, so try to avoid it. As far as hopping curbs - it is possible - although it takes practice and a bit of dexterity.
I second the above posts about buying sight unseen. don't do it - go to your local bike shop (LBS).
Make sure they treat you well - You should get the following deals when you buy a bike:
- Free service for at least a year (this is important - your bike will need more frequent tuneups in the first year, and it also shows that the shop takes care to adjust their bikes properly, so they don't have to waste time doing it over again.)
- Free fitting
- 10-20% off accessories is a nice bonus - it's worth asking
If your lucky your local shop may have some used bikes - also worth the inquiry.
Creating a good relationship with your local shop is very advantageous. If you are a loyal customer, and you show some interest beyond "it's broken, make it work" you should be treated very well.
About a year ago I bent my front wheel running into a car. I walked over to the bike shop I use and the owner gave me a new wheel for $20 - tire included - the tire by itself was probably worth more than $20, but he saw I was in a pinch, he knew I was a good customer, so he gave me a good deal. And I rode home, rather than waiting a week for a new wheel to show up after I bought it off of ebay.
Especially as a young person or college student you will be treated well - especially if you show some interest in the sport. Ask about local group rides or races - people love to see young folks getting involved in this sport.
As far as speed goes, I am going to estimate that getting on a road bike for the first time you'll average 15-17 mph on the flat cruising at a relaxed tempo.
That's all the questions I can remember.
if you haven't been there yet go to
http://www.sheldonbrown.com/
There are more answers there than you could possibly absorb in a lifetime.
good luck