Before shopping for a heart rate monitor it is important to determine how and where you will use it.
Most monitors consist of a chest strap that reads your heart rate and a receiver, usually built into a watch. There are some monitors that use mp3 players to display the pulse rate and others have a finger pad for reading your pulse.
Consider the type of training and exercise that you will be doing. If you are training for a triathlon for example you will want a monitor that you can use swimming as well as running and cycling. In this case you would want a waterproof system with a constant monitor such as a chest strap.
If you are like me then most of your use for a heart rate monitor will be on a piece of fitness equipment such as a treadmill or elliptical trainer.
For those of us who are planning on using the monitor with a piece of fitness equipment, see if the monitor is compatible. Many of the newer pieces of fitness equipment have built in receivers for popular heart rate monitors. This can be an issue in a gym however! I know in my local gym there is sometimes cross talk between adjacent equipment. My heart rate will often display on the treadmill next too me as well as my own.
One of the biggest differences between different heart monitors is the amount of data that they will process, store and transmit.
Your basic monitor will display your heart rate and either beats per minute or as a percentage of your target heart rate. Because most receivers are contained in a watch they will most likely have sports watch features such as a stopwatch.
More advanced models will calculate the amount of time during each workout that you were in the target heart range as well estimating the number of calories expended.
Some monitors will even connect to your computer. Data will be transmitted and stored to track your progress.
Most importantly make sure that you can easily read and use the monitor. Like any exercise equipment, it won't help you get and stay in shape if you won't use it!