Cadence vs. Speed

Cycling computers have two different settings, so obviously there's a difference. However, the terms are often confused, as they are related. While speed is strictly a measure of the distance covered per unit time, cadence is a measure of the number of revolutions of "something" per unit of time. In cycling, the "something" generally refers to the crank, whereas for running it's applied more to footfalls. Basically, in mathematical terms:

Cadence (revolutions per minute) * Distance (per revolution) = Speed (distance per minute)

For cycling, the distance per revolution is equal to the circumference of the wheel, whereas for running it is a measure of stride length. To illustrate the difference between the two, I'll leave you with these two examples:

• A cyclist riding a mountain bike will need to pedal faster (cadence) in order to keep up with (speed) a cyclist riding a road bike because mountain bike tires are smaller.
• A shorter runner and a taller runner are racing eachother. If their stride rate (cadence) is the same, the taller runner always wins (speed) because they can cover more distance with each stride.