When thinking about the type of bike riding you're most likely to do, if you're going to be riding exclusively on pavement and want to go pretty fast and/or ride long distances, a road bike is probably what you want. Designed for racing, road bikes typically have a lightweight frame which is designed to allow the upright rider to maintain the most aerodynamic position possible.
Wheels: A road bike typically has narrow, smooth high pressure tires that minimize contact with the road to provide the least rolling resistance possible. As these types of tires allow you to feel each bump and pebble in the road, it's not necessarily the most comfortable ride, but that's not the intent.
Frame: The material used in road bike frames, like most other bikes will vary depending on their cost. In general terms, the more expensive a frame is, the lighter it will be. Having a lighter bike is most important in climbing, though it also factors into downhills and riding in the flats. A heavier bike usually translates into somewhat slower times for competitive riders, though that may not be important to you.
Most entry level bikes have either steel ("cro-moly") or aluminum frames these days, though I'd venture that aluminum is becoming the predominant material for basic road bike frames. There is certainly nothing wrong with either choice, but there are trade-offs. For instance, for aluminum to be strong enough to be durable over the long haul, the frame must be pretty stiff, which may translates into a slightly rougher ride. Steel may be heavier, but can flex more at the same weight, which can cushion the bumps a bit. The best way to find out which is right for you is to ride several different bikes and see if you can tell a difference.
As the price of the road bike goes up, you’ll start to see components (such as the front fork) switching from aluminum to carbon. Finally the whole frame on higher end bikes will be made of carbon fiber, which is very strong and yet light weight at the same time. The frames on the most sophisticated and high performance road bikes are made from space age materials like titanium, which is both amazingly strong and light. Because of the expense of these materials, these bikes typically fall beyond the range of all but the most serious or competitive cyclists.
Handlebars: Road bike handlebars go out straight from the stem and then curl under, allowing riders who want to go really fast to hunch over when riding at that point, in order to reduce wind resistance. This is known as going into the drops. Riders can also sit in a more upright position, with their hands on the flat, top part of the handlebars. Typically, you'll find both the brake levers and the gear shift levers mounted on the handlebars of a road bike for easy control.
Riding position: The way that a road bike is designed allows riders to bend far forward, reducing their profile and cutting down on wind resistance, along with putting them in a position to drive maximum power from their legs and hips through the pedals. While aerodynamic, being hunched over like this for any length of time may not be the most comfortable position for some riders as it requires you to support a substantial portion of your body weight with the upper body. This can cause strain and soreness in the hands, wrists, shoulders and neck if a rider is not used to it, or is riding a bike for which they might not be properly fitted or sized.
Gears: Road bikes have a wide range of gearing, with low gears that allow a rider to more easily climb steep hills up through rather high gear choices that a rider uses to go really fast. Typically a road bike will have either two or three chain rings as part of the crank assembly in the front along with eight or nine gears in the cassette on the rear wheel. This combination allows for anywhere from 16 to 27 possible gear combinations, a broader range than yesterday’s typical ten-speed afforded.
Pedals: Basic road bikes may come equipped with platform pedals. If this is the case, toe clips are usually a standard accompaniment, or may be added quite cheaply. However, clipless pedals are frequently/usually used on road bikes, which allow the rider to clip his or her cleated shoes to the pedals, providing the ability to drive the pedals through the full rotation of the circle, pulling up on the pedals during the upstroke as well as pushing them down.
Typical road bike accessories: cyclocomputer, frame pump, tool bag, water bottle and cage.
Major brands: There are almost as many road bike manufacturers as there are riders. Major manufacturers include Specialized, Trek Bianchi, LeMond and Cannondale.
Buying advice: If you are buying a new bike and are reasonably certain you're going to stick with it for a while, buy the best frame you can afford, and if you have to cut corners to save some money, do it on the components. You can always upgrade them later. Buying a bike with a junky, heavy frame to save a few bucks will usually prove unsatisfactory, and you'll end up buying a whole new bike later, rather than being able to improve it piece-by-piece as your desires and wallet allow.