This article is from the Fitness FAQ, by Jeff Gleixner (firstname.lastname@example.org) with numerous contributions by others.
Heart Rate (HR) is the number of times the heart beats per minute.
This is usually taken by pressing on the front, left side of the neck,
or the inner wrist, and counting the number of beats in a certain time.
i.e. If you counted 10 beats, starting at zero, in 10 seconds your
HR would be 60 (10*60 sec(1min)/10). When doing an aerobic work out
your Max Heart Rate (MHR) needs to be determined. Your MHR is defined
as 220-Your Age. A 20-Year old's MHR would be 200, while a 50-Year
old's would be 170. While performing aerobic exercise your HR should
stay in the range of MHR*.50 to MHR*.80. This is your Training Zone.
So a 20-year old should keep their HR between 100 and 160. Keeping
the HR above this zone will not provide additional aerobic benefit,
in fact, it may provide no aerobic benefit at all. For best results,
aerobic and FAT burning, keep your HR in the aerobic zone for at least
12 minutes. The longer and more frequently you do this the more
improvement you'll see.
Read "Fit or Fat" by Covert Bailey,listed in the books section, for a