The first thing to do before purchasing a bicycle for any reason is to know what your primary riding is going to be. Most people initially purchase a dual use bike, something that can be ridden on road and off road. Eventually, when the bug bites big it’ll be time to invest in a bike that is all muscle for the mountains.
One of the ways that mountain bikes differ from other bikes is that they have very strong, lightweight frames, multiple gears, powerful breaks and wide tires. A comfort bike, or recreational weekend bike, touring bikes and cruiser bikes are built more for comfort than durability. WIthin the mountain bike category there are several types of bikes available; the cross country bike, the trials bike, the downhill bike and the jump/slalom bike.
Generally the largest percentage of mountain bikes sold are in the cross country, or XC classification. These are good dual purpose bikes. They are lightweight and good for riding over tough terrain, but they are also comfortable for road riding.
Jump/slalom bikes are also good dual purpose bikes. They are very strong and have an excellent front suspension.
Downhill and trial bikes are...