Heart rate monitors are electronic devices that usually take the form of a watch with or without a chest strap. The main idea with these is that your heart rate gives you the best information about how hard or intense your training session is
and with the right monitoring you can train in the correct zone for maximum results in fitness and weight loss.
The next step up from a heart rate watch or monitor is getting a GPS watch that uses satellites to accurately track your exact position over a training session. These watches are very clever and use the GPS (global positioning system) satelillite system to give you exact your position anywhere in the world! Neat huh?
The great thing about this technology is that you know exactly how many kilometers you've walked or run on a training session to within the meter. No more driving back over your course to see whether you broke your 5km PB, the watch tells you. Are they accurate? You bet! The best brands will only be 1-2m out over a 1000m distance so it's pretty much spot on.
*text taken from http://www.completefitness.com.au/articles/fitness/fitnessgadgets.php (alot of great articles to check out along with the above text)