|Quoting Kaddyuk (Reply 1):|
A Mountain Bike can be used on both road and terrain...
Although this is true, doing so would be similar to going running in hiking boots. Can it be done? Sure. The difference, however, is immense.
Mountain biking and road biking are two very different sports.
Mountain biking is a sport in which you suit up and go into battle. There is a lot of struggling, aggressive terrain that must be negotiated, and because the rider must constantly adjust his/her weight distribution, it tends to be more of a total-body workout. In addition, mountain biking tends to resemble more of an interval workout, in which the rider works very hard while climbing, catches their breath while descending, and tackles a climb all over again.
Road biking, on the other hand, is comparitively tranquil, and can be compared to running. On one hand, it can be physically demanding, but on the other hand, the rider can relax, clear their mind, and collect their thoughts. Road biking is also much less abusive on the rider's body, both in terms of there being less impact and beating, and in terms of there generally being fewer crashes overall.
It's important to bear in mind that these aren't the only two segments of cycling. Road is one extreme, and mountain is another. There is an extremely popular segment of bikes in between, known as "pavement" or "urban" bikes.
This segment is designed for people who want to get into fitness riding on the pavement, with some cross-training on dirt roads and groomed paths. The bikes are variations on "hybrids", but are tuned toward lighter weight and higher performance. They can be considered lightweight, fast mountain bikes, or beefy road bikes with flat handlebars and wider tires.
The nicest bikes of this category (in my opinion) are Trek's FX
You can check them out here:
Prices (all I know are USD) range from about $350 to $1200. The 7.5FX pictured above has really nice, lightweight wheels, and retails for about $650. It's also available with disc brakes for better all-weather braking.
Hope that helps...