Mountain bikes have low low gearing, the lowest of any style of bike, good for a steep hill. They have big tires which give a soft ride. Modern, good mountain bikes usually have an oil-damped suspension fork, and to get that you need to shell out about $500 for a budget brand or more for a premium brand. Most of them will now have disk brakes, the better ones have hydraulic disk brakes. There are several different styles of mountain bike but the type you see commuting is almost always a "sport" bike which is almost the same thing as a cross country bike, only cheaper and heavier. Mountain bikes have some disadvantages on the road, chiefly their knobby tires but also their weight and suspension. Most of their advantages on pavement are also shared with "fitness hybrid" style bikes, which have larger wheels, in-between size tires, and usually no suspension, but have gearing nearly as low and about the same posture.