Some things to think about when buying a road bike:
Road bikes tend to last awhile, so it's important to get one you're going to be happy with for 5 - 10 years. The main things to think about are Price, Components, Frame and Fit:
1. Price - you can buy a decent road bike for anywhere from $500 - more than $2,000, depending mainly on how new or used it is, the frame, components and wheels. I'd say anything over $1,000 is probably a pretty good bike. It will help to first figure out how much you want to spend.
2. Components - The most common compenent manufacturer is Shimano and you can tell a lot about a bike by which of the Shimano line of parts it has on it. Once you decide which component level you want, most of the other decisions will fall into place around that. Shimano has 5 major lines:
Dura-Ace- Top of the line, for extremely serious riders and racers willing to spend the $$. Typically come on bikes that are $2,000+
Ultegra- Very good compoenents, smooth, will last forever, great value, most serious 'weekend' riders use Ultegra components. You can probably get a good bike with Ultegra compents for around $1,500.
105 - New bikes in the $1,000 range will come with 105 components. 105 parts are solid and will last, just a bit heavier and not quite as smooth as Ultegra components.
Tiagra- Lower quality than the 105 parts, a good package for casual riders on their first bike. Usually on bikes that cost less than $1,000 new.
Sora- These are the lowest quality components of the line, ok for very casual riders.
3. Frame - The most common frame materials are aluminum, steel, carbon and Titanium.
Aluminum: This is the most common material because it is lightweight and inexpensive. Aluminum is a very stiff metal with very little natural flex. This makes it ideal for racing, as very little power is lost in each pedal stroke, but it can feel a bit stiff because of the lack of flex. Also the stiffness can make the ride a bit tougher on your body because you feel the vibrations in the road more. Aluminum frames have a shorter lifetime because if they bend they can oxidize and can't be bent back. That being said, if you are racing or just getting into biking, aluminum is a good choice.
Steel: Steel is expensive, expect to pay at $800 or more for a good steel frame since it the engineering to get steel to be light enough to ride on is very costly. Steel has much more natural flex which allows the frame to absorb much of the roads vibration. Anyone who spends long hours on the road will come to appreciate the value of steel, and it is no surprise that riders of steel frames are so passionate about their frames.
Carbon Fiber: Carbon Fiber can have different amounts of flex depending on how the fibers are laid. Carbon frames can have varying amounts of vertical flex (that absorve vibration) without creating lateral flex that can give the bike an unstable feel at higher speeds. They are great lightweight frames, that are typically very expensive.
Titanium: Titanium combines the lightness of Aluminum with the flex of steel. Titanium is considered the top of the line for frames and can cost more than $2,000 just for the frame.
4. Fit - This is probably the MOST important thing about your bike. The two main parts of fir are height and reach. Most good bike shops will fit your bike to you when you buy it. This will make a huge difference in your comfort and riding experience.
Height: When you stand over your bike and pick the bike up by the seat and handle bars, there should be enough room (ideally about an inch) between your groin and the bike to lift both wheels off the ground.
Reach: While riding, your weight should be balanced between your hands and yoru rear. Your weight can be shifted by either raising the handlebars (from the stem) or pulling the handlebars back (either by getting a shorter frame, or adjusting the stem).
Try riding a bunch of different bikes before you buy one and see which feel more comfortable for you. Have fun!