There are several variations of lightweight performance cycling. These include ‘Road’, ‘Time Trial’, ‘Triathalon’ and ‘Cyclocross’. These are all variations of the same theme: Go faster!
Road
Many people equate road riding with any sort of riding on a road. In traditional cycling parlance, it actually refers to a particular class of high performance racing. The Tour de France is the highest-profile ‘road’ race in the world, and is the exemplar of this style of riding.
A typical road bike is lightweight, delicate and streamlined. It usually has very 
skinny, very high-pressure tires and ‘drop’ style handlebars. The gears on the rear are small (compared to a mountain bike or hybrid), with only a slight difference between one gear and the next. This allows experienced racers to maintain their cadence, whilst fine-tuning their gear ratio.
Road bikes are great for going as fast as you possibly can, particularly in a group, over moderate to long distances and for building overall endurance, leg strength and core strength.
Road bikes are not optimal for touring or commuting, as they are rarely equipped to accommodate fenders or luggage racks, and are not terribly rugged.
They should never be used for cross-country riding. Neither the frame nor the wheels are built to take the kind of abuse this type of riding entails. If interested in riding a road-type bike in rough terrain, take a look at the section on ‘Cyclocross’ towards the bottom of this page.
Time Trials
Whereas in ‘road’ racing, competitors ride in packs, often as part of a team, 
and draft one another, time trials are all about individual performance. In time trials, there is no drafting, and there are rarely any hills, so gearing is higher and aerodynamics play a greater part then weight. Time trials bikes usually have either very deep wheel rims, or disk wheels. The frame often incorporates airfoil design, and the handlebars do not drop down (like ‘drop’ bars). In most cases, the bike is also equipped with an ‘aero’ bar, to promote superior aerodynamics of the rider (at the cost of stability).
Time trials bikes are great for physical training, and for high-performance solo riding. They are not ideal in most other roles.
Triathlon
Triathlon bikes are very similar to time trials bikes (so much so that manufacturers will often build time trials bikes using Tri frames). Triathlon bikes usually have more versatile gearing, because the routes tend to be longer and hillier. The handlebars and stem are usually below the level of the saddle, affording a very aerodynamic position, as well as allowing the rider to rest the ‘running’ muscles (the running portion of a triathlon is directly after the cycling).
Much like time trials bikes, triathlon bikes are very specialized, and are not easily adapted to other roles.
Cyclocross
What do you do when there’s an obstacle in the way of your ride? Go around, right? Not if you’re a cyclocrosser. Cyclocross, originating in western Europe, is a form of racing which involves maneuvering through rough terrain and sharp turns, and overcoming obstacles. In a race, riders will often have to ride through sand, mud, grass and loose gravel. In addition, courses usually involve carrying the bicycle while clambering over obstacles (such as walls, fences, fallen logs), and running up stairs. 
All this requires a versatile, lightweight bicycle. A cyclocross bike has a lightweight frame, and drop bar handlebars, similar to a road bike. It also has similar gears to a road bike. A ‘cross bike has beefier wheels (more akin to hybrid wheels) and wider, knobbier tires. The frames and forks are wider around the wheels, providing greater clearance so that mud and debris are less likely to gum up the works. The only brakes that will function on a cyclocross bike are cantilever or road disc brakes. Cantilevers have been the standard brake on cyclocross bikes until very recently, and are certainly the lightweight alternative, but lack in stopping power. Road disc brakes are more powerful, but have not, until very recently, been permitted in cyclocross racing. Some ‘cross bikes do come ‘disc ready,’ though, allowing commuters and casual cyclocross riders to make the switch.
Cyclocross-style riding is a great way to cross-train. In a single kilometer of a race, a rider may be sprinting, climbing, carving, slogging through sand and running up a steep incline carrying a bike.
Many commuters have started riding cyclocross bikes. They are light and sturdy, can be easily equipped with good fenders, and many have frames that will accommodate luggage racks and road disc brakes. These bikes are often a solid alternative for those who want a decent commuter bike, but don’t want to give up the drop bars. Perhaps, now that road disc brakes are ‘legal’, we’ll start to see ‘Cross bikes come standard with them.
