The cleat mounting surface is different. You'll want to get shoes that match your pedals.
Typical road bike shoes have a flat sole and are drilled for a 3 bolt, Look style, cleat mounting. Mountain bike shoes generally have a recessed 2 bolt, SPD style, mounting surface.
Mountain bike shoes and cleats are easier to walk in. Road bike shoes have a larger contact surface area and are less likely to cause "hot spots" on your feet while you are riding.