Mountain bikes are designed for off road riding and come in many levels of quality made for different terrain and rider skills. Frames and components are built tough in order to handle rocks, roots, logs, and anything else that off road trails will throw at you.  Wide tires with a knobby tread will allow you to go over all these obsticles and will give you traction even in wet muddy conditions.  Mountain bikes are also fine to ride on the road, and tires can be changed to one that is more road oriented.