Mountain bikes are designed for off-road riding. Whether made for racing or the more leisurely rider, there are four basic frame designs for mountain bikes: rigid, hard-tail, soft-tail, and full suspension.
The rigid (also commonly known as fully rigid) mountain bikes resemble a conventional sort of bike as you would imagine it. A rigid mountain bike has neither a rear suspension nor a front suspension. These used to be the most common kind of mountain bike but they are rapidly being replaced by the more comfortable models which feature some sort of suspension system. Fully rigid bikes tend to be lower-priced than models with a suspension. Many riders still prefer rigid bikes because it's what they're accustomed to, and the feel that the rigid design allows them greater control.
Another common mountain bike design is the hard-tail. Hard-tail mountain bikes are so named because they have a conventional rear end without a suspension. Hard-tails differ from fully rigid bikes in that they have a front suspension. The hard-tail is perhaps the most popular mountain bike design. Many professional riders prefer the feel of a hard-tail and the comfort of a front suspension.
The next kind of mountain bike design is the soft-tail. As you probably guessed, soft-tails employ a rear suspension. Rear suspensions are a more recent innovation than front suspension due to the complexities of the design. Soft-tails are often more expensive than hard-tails, but they also tend to be more forgiving.
Finally there's the full suspension mountain bike. Bikes with this design feature both front and rear suspensions. These bikes are the latest mountain bike design. For a long time riders avoided full suspension bikes because they were heavy and they tended to sag in the middle like an old swayback horse. It took years for engineers and riders to perfect a viable, functional full suspension mountain bike. It was in the 1990's when these bikes finally reached a point where they were marketable.
In addition to the frame and suspension, there are other differences in mountain bike designs. One key area where bikes differ is the brakes. Mountain bikes either have some form of caliper brakes, which are the kind of brakes typically associated with bikes. This kind of brake squeezes the rims to slow and stop the bike. The other type of brake is the drum brake. Drum brakes on bikes are similar to the braking system on automobiles. Drum brakes are more expensive than caliper brakes but they're also easier to maintain. Mountain bike braking systems have even been made which utilize hydraulic power.
Another area where you have some choice in mountain bike design is tire size. Mountain bike tires typically range in size from 24" to 29". Smaller tires are used when greater maneuverability is required as in stunt-riding and jumping. Larger tires are used when speed is the key.
As with any other important purchase, do your homework before buying a new mountain bike. Think about your needs and your budget. Mountain biking is a terrific past-time. It's a fun way to get some fresh air, enjoy the outdoors, and improve your health.