HRM

You use a heart rate monitor during your work out to make sure you are working out in your target zone (aerobic, anaerobic)

Use it to determine your resting heart rate. Put it on, start it tracking, then lay down and be still for a few minutes, then check your heart rate.

Now go here: http://www.briancalkins.com/HeartRate.htm
Put in your info and you'll get a table (the Karvonen formula one)

To be getting a good aerobic workout, you want your heart rate in the 60-75% heart rate range
For anaerobic, above the 75% range