In cycling, cadence refers to the number of revolutions per minute (rpm) of the pedal set. To find your cadence, pick a leg and count the number of down strokes you make with that foot for 15 seconds, then multiply that number by four. A cadence between 60 and 80 rpm is considered healthy for your knees, with experienced cyclists riding a bit higher in the 70 to 90 rpm range. Elite cyclists tend to spin even faster; Lance Armstrong can ride at around a 120 rpm cadence. Riding at a higher cadence uses more slow-twitch muscle, which is great if you're riding for endurance. Slow-twitch muscles are more resistant to fatigue than their fast-twitch counterparts, but they don't have a lot of power, so using a lower gear but pedaling faster means you can ride longer. Plus, slow-twitch muscles primarily burn fat for fuel.
You can easily track your cadence with a bike computer like the SpeedZone Analog Elite Computer ($30) by Specialized.