A pedometer is a pagersized device worn on your belt that simply records the number of steps you take based on your body's movement. Some pedometers are analog devices that simply measure steps. Some are fancier digital models that track the distance you walk, plus the calories you burn. But keep in mind, the calorie counters are notoriously inaccurate and those models are more expensive. All you really need is a simple step counter so you can monitor if you walk the recommended 10,000 steps per day.
A pedometer is great for the gadget lovers out there. It's also ideal for the person who simply can't find a hunk of 30 minutesor even three chunks of 10 minutesin a day for walking. Just be sure to put a safety string through the pedometer's waist clip and pin it or loop it through a belt loop, so the pedometer isn't dropped down a toilet.

If you want to know not just the number of steps you've taken, but the distance as well, you can calibrate a pedometer. The simplest way is to wear it while walking a known distance, such as once around a quartermile track, at your normal walking speed. Then multiply that number of steps by four, and you know your typical number of steps per mile. (For greater accuracy, you should walk a full milefour times around the track). Now, anytime you want to estimate the distance you've walked, just divide the total number of steps you've taken by your "steps per mile" calibration. Keep in mind it's just an estimate, because the length of your stride increases as you walk faster. So, on faster walks you'll be underestimating the distance somewhat, and on slower walks you'll overestimate a bit.
Some pedometers allow you to enter your step length (based on a calibration walk) and they will calculate your walking distance automatically. Fancier models will even estimate the calories you burn if you enter your body weight as well. But don't count on these calorie estimates to be particularly accurate, given the wide variation of fitness levels and personal physiology of individuals.
Example:
Jan wears her pedometer for a walk around the quartermile school trackit counts 473 steps. She multiplies by four, to estimate that she takes about 1,892 steps a mile. (For easier math, she calls it 1900 steps.) Another day she takes a walk and covers 6,685 steps. Jan divides 6,685 by 1900, and gets 3.52, or about three and a half miles walked.
To calculate a step length, divide the known distance you've walked in feet by the number of steps you've taken. A quarter mile walk is 1,320 feet long (a mile is 5,280 feet). So Jan divides 1,320 feet by her 473 steps, and learns each step is 2.79 feet long. Now she can enter that in the pedometer.