Types of Bicycles: Road Bike
 

The Road Bike 
A very light weight frame, dropped (curved) handlebars, narrow seat, and very narrow tires. This design originated in European bike racing, and just about every feature comes from that fact. The dropped handlebars and narrow saddle allow the rider to pedal efficiently while keeping his/her back nearly parallel to the ground. This position is fast because it allows the rider to maintain the most aerodynamic position possible.

Wheels
The wheels and tires are also chosen for performance. Narrow wheels and tires are lighter and have less rolling friction so you go faster with less effort, but the tires transmit every bump in the road and are more prone to flats. If you want to average 15-20 MPH or more, then a good road bicycle is the answer as the weight, frame design, narrow wheels and lightweight components are designed for going fast.

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. Most entry level road bikes have either steel, aluminum or (now) carbon fiber frames. There is certainly nothing wrong with either choice, but there are trade-offs. Aluminum frames tend to be stiff, which may translate into a slightly rougher ride. Steel may be heavier, but can flex more, which can cushion the bumps a bit. Carbon fiber is typically the most expensive of the three, but it will be lightweight and provide good stiffness balanced with ride cushioning. The best way to find out which frame material is right for you is to ride several different bikes to experience the difference. 

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.

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 to ten gears in the cassette on the rear wheel. This combination allows for anywhere from 16 to 30 possible gear combinations.

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.

Buying Advice
If you are buying a new or used road bike and are reasonably certain you're going to stick with it for a while, buy the best frame you can afford. You can always upgrade the components later. 

You can search for a new or used road bike here.