Mountain bikes and road bikes are both types of bicycles designed for distinct uses.
Mountain bikes are created for off-road cycling in non-paved rough environments. Mountain biking became a sport in 1970s and has since then diversified into cross-country riding became an Olympic sport in 1996. Less popular as difficult to film and televise and is the only mountain biking event at the Summer Olympics.
Road bikes are primarily used on paved road surfaces, mainly only as conveyance.
Cross country biking is classified by the type of terrain: rough forest paths, smooth fire roads and single track. Downhill mountain biking is a gravity assisted timed race. Downhill races are held on steep, downhill terrain with no extended climbing sections, resulting in high speed descents with extended air time off jumps and other obstacles. The original concept of free-riding was that there were no set courses, goals or rules to abide by. The original free-ride bikes were modified downhill bikes which utilized gearing that enabled the rider to go up hills as well as down them. Modern free ride bikes are similar to downhill bikes, but feature slightly less suspension travel and are lighter - which enables them to be ridden not just downhill but through mountain paths.
Touring road bikes are designed to take weight and luggage while being comfortable with the bike frame favoring rigidity over flexibility, heavy duty wheels to increase the load capacity and multiple points for attaching rucksacks, water bottles and fenders. Hybrid road cycles are utility cycles that can be used for both city riding and commuting on paved roads and had off-road capabilities. Utility bikes are the most commonly used bikes globally and is used for simple community and running errands.
There are different designs of bikes for the optimal types of uses. Mountain bikes have wide knobby tires for good traction and shock absorption, front and rear suspension and up to 30 gear speeds to be used on a variety of terrains. Free ride bikes have less emphasis on weight and more on strength. Downhill bikes have more suspension travel. They are built with different bike components, e.g. frames that are strong, yet light, which often requires the use of more expensive alloys and can be as light as 40 pounds.
Road bikes have light weight aluminum frame with drop handlebars, and high pressure narrow tires to decrease rolling resistance.
"Mountain Bike vs Road Bike." Diffen.com. Diffen LLC, n.d. Web. 24 Oct 2014. < http://www.diffen.com/difference/Mountain_Bike_vs_Road_Bike >