Both mountain bikes and beach cruisers have two wheels and a frame, but they are designed for riders with different purposes. Ride a mountain bike on just about any surface -- they're built with performance and durability in mind. Beach cruisers are comfort bikes that can't handle rugged terrain. Cruisers generally have a large, cushioned seat and can get you around town comfortably in style. The tires, components and gearing differ greatly between the two bikes.
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.