A single-speed bike has one gear, whereas a multi-speed has considerably more, though not usually more than 21. Lower gears trade speed for power, which is useful for negotiating hills, grass, and rough terrain. Higher gears do the opposite, making them suited for speed over level terrain. A multi-speed bike will let you do more, but the single-speed, by virtue of simplicity, is more reliable and in many cases, less expensive. BMX bikes are single-speed, road bikes and mountain bikes are multi-speed..