Heart rate monitor
A heart rate monitor is a device that allows a runner to measure his or her heart rate in real time. It usually consists of two elements: a chest strap transmitter and a wrist receiver (which usually doubles as a watch). Strapless heart rate monitors are available as well, but lack some of the functionality of the original design. Advanced models additionally measure heart rate variability to assess a user's fitness.
The chest strap has electrodes in contact with the skin to monitor the electrical voltages in the heart. When a heart beat is detected a radio signal is transmitted, which the receiver uses to determine the current heart rate. This signal can be a simple radio pulse or a unique coded signal from the chest strap; the latter prevents one user's receiver from using signals from other nearby transmitters (known as cross-talk interference).
There are a wide number of receiver designs, with all sorts of advanced features. These include average heart rate over exercise period, time in a specific heart rate zone, calories burned, and detailed logging that can be downloaded to a computer.
The Polar Electro company's website states they invented the first accurate, wireless electrocardiogram heart rate monitor in 1977, to be used as a training tool for the Finnish National Cross Country Ski Team. 
How to train with a heart rate monitor
Although there are entire books written on this, the basic principles are 1) determining your maximum heart rate, and 2) scheduling workout with specific target heart rates. Some runners hire an exercise physiologist to determine maximum heart rate by administering a treadmill test. In the absence of a test, a number of formulas can be used to approximate a maximum heart rate. One easy formula is:
Maximum heart rate (men)= 220-your age
Maximum heart rate (women)=226-your age.
A formula for more conditioned, active runners is:
Max heart rate (men) = 214 - (0.8 *age)
Max heart rate (women) = 209 - (0.7 *age).
Next a runner should measure his/her resting heart rate using the heart rate monitor immediately after awakening and before getting out of bed. Target heart rates for specific workouts can then be set using the formula:
Target heart rate =( (Max heart rate-Resting heart rate) * Workoutpercentage ) + Resting heart rate,
where the Workout percentage is 85% for a hard workout and 70% for an easy workout.
How to race with a heart rate monitor
Calculate a minimum target and maximum target heart rate based on the 70% and 85% levels described above and then use the alarm feature of your heart rate monitor to alert you if you leave that range. Try to keep a steady level of effort throughout the race, but avoid having the heart rate monitor distract you to the point where you are not focusing on the race and on how your body feels.
Note that as a runner's fitness level improves, the resting heart rate will decline.
- John L. Parker, Jr., Heart Monitor Training for the Compleat Idiot Cedarwinds; 2nd edition (August 1998) ISBN 978-0915297252