There are literally hundreds of variating levels of different bikes now in Mountain bike, and it is vitally important to get the right choice of bike for the right job for you. It also helps to make sure your money stretches as far as it can to cover all bases you require at whatever level of cycling you may be at.
Mountain bikes have wider wheels and a more easier riding position and allow you to go over more terrain. They will have front or full suspension and the higher the specification of bike the lighter they get and the faster they ride. Mountain bikes only until recently had 26" wheels but now come in 26", the more competition friendly 27.5" and the ever popular 29" which has transformed the mountain bike market with smoother rolling wheels.