From their targeted terrains to overall weight, handlebars and wheel types and much more, there are fundamental differences between road and mountain bikes.
A road (or racing) bike is targeted at exactly that; for riding on the road at speed. They’ll usually boast thin 700c-sized wheels with slick tyres, a lean overall frame weight (often using carbon at higher price points) and generally thinner frame tubing (except on some aero road models) compared to a MTB.
The drop handlebars, with a flattened top section and each end curving forwards, down and back in again, and overall geometry of the bike (i.e.the increased distance from seatpost to handlebars) have a more aggressive geometry than a mountain bike (and also hybrids) to encourage better aerodynamics by reducing the frontal profile of a rider.
While you can happily ride a mountain bike on tarmac, they excel at riding off-road, from towpaths to technical trails and on mud, girt, gravel and more. As well as versatility, a mountain bike will provide added durability with its reinforced – if heavier – frame materials, a larger saddle, wider and tougher wheels (sizes range from 26” to 27.5”, 29”, Plus and Fat, but that’s a whole other story), and thicker tyres with lugs for off-road traction.
Mountain bikes feature a flat handlebar with the brake levers on the top and the geometry promotes a less aggressive riding position for improved comfort and better handling when dealing with tricky trail sections.
Also key for mountain bikes are their suspension systems (either hardtail with just a front suspension fork, or softail/full suspension with both front and rear suspension abilities) for reducing the roughness of the terrain. Seatposts can often have in-built suspension on a MTB as well.