1. Full-suspension or hardtail: Full-suspension mountain bikes have suspension components in the front fork and in the rear of the bike, whereas hardtail bikes only have a front suspension. Most average mountain bike riders, and many serious riders, prefer hardtail mountain bikes. Hardtail mountain bikes tend to be lighter, more durable and cheaper than comparable full-suspension mountain bikes. Full-suspension bikes give maximum shock absorption and comfort while you're riding rocky trails.
2. Bike size: The frame size of a mountain bike isn't as important as you might think. You will typically want a mountain bike that is a few inches shorter than a road bike. Most men will want a mountain bike that measures 18-21 inches. Most women will fit well on a mountain bike that is 16-19 inches. The reason the size of the frame isn't critical is that the other components (handle bars, seat and pedals) can be adjusted to give you your optimal riding position.
3. Brakes: The two styles of brakes available on mountain bikes are mechanical pull brakes and disc brakes. Disc brakes usually provide greater stopping power and work better when mountain biking in wet, muddy conditions. Mechanical pull brakes are much more affordable.
4. Shifting: The two main styles of shifting mechanism are lever style and twist style. Twist-style shifting mechanisms are usually located on the inside part of each handle and require you to twist them forward or back. Lever-style shifting mechanisms shift when you flick a lever forward or back. Both work in nearly identical ways, and your choice will be based on personal preference.