Mountain bikes are designed for off-road riding. They have multiple gears for riding up and down hills, paths, washes, and trails of varying inclines. To deal with the sand, dirt, mud, and gravel commonly encountered off-road, these models are equipped with wide tires with knobs to provide greater traction. Rather than curved handlebars, mountain handlebars are generally straight. The rider generally sits upright while riding this style.
Hybrid bikes are probably the most common style seen today. They are a combination of mountain and road bicycles, making them well suited for multiple terrain types. The handlebars are straight and the seating position is upright, just like a mountain model. The frame will usually be lightweight, the tires narrower and smoother, which makes for smoother, easier rides on pavement. As with the two styles it has evolved from, a hybrid usually has multiple gears.
BMX (bicycle motocross) bicycles are very popular with stunt riders, certain types of off-road racers, and youths. The BMX style is generally for a smaller frame bike, built robustly to handle hard landings and other stunts. The front wheel and handlebars can be rotated 360 degrees to allow for tricks, and the models will only have one gear. Due to the smaller nature of the frames, riders will generally stand while pedaling.
Cruiser models are designed for ease of use and a comfortable ride. As a result, they are popular with adult riders who are new to riding, or only do so occasionally. Cruisers offer an upright seating position and a fixed gear, so the rider simply has to sit down and start pedaling. Their wide tires make them comfortable. Due to their heavy steel frames and single gear, they are generally rather slow compared to most other types.