Mountain Biking typically refers to riding a bicycle off paved surfaces, commonly flat rock surfaces, or fire roads or "single track" paths in more heavily forested areas.
Mountain bikes differ from road bikes in many ways. A mountain bike's lowest gear is usually lower than a road bike's lowest gear. Mountain bikes have wide, knobby tires that can be run at lower pressure (35 - 50 psi is common). The handlebars on a mountain bike stick straight out from the stem, but can be built with a slight rise. Most mountain bikes have a front suspension, which helps absorbs the shock of hitting common obstacles on trails, e.g. roots, rocks, and holes. Many mountain bikes also have rear shocks, which can provide a smoother ride, but increase weight, and are considered less efficient over smooth ground.