Mountain bikes come in a variety of models, some of which can handle only mild, rocky terrain, and others designed to handle jumps off cliffs. If you go for high-performance models, you'll get a light frame, heavy-duty shocks and thick tires with a deep tread to keep you stable on rough trails. The lower-key mountain bikes feature an upright seating position, much like cruisers, have heavier frames and comparatively more comfortable saddles. Mountain bikes usually have a detailed gearing system with 15 speeds.
A cruiser has a relatively heavy frame, but because it has no shocks or significant tread on the tires, it isn't designed to handle terrain beyond city streets and sidewalks. You sit upright on a cruiser, and the handlebars are situated higher for comfort, rather than aerodynamics. You typically get between just one and three speeds on a cruiser. Cruisers have balloon tires -- large, fat tires that contain minimal air pressure -- making for a cushy ride. These bikes are easy to handle and work well for casual rides.