There are TONS of types of HR monitors, don't get bewildered by the many models. I like Polar, mostly because they are the oldest manufacturer, and they have a good warranty program.
You don't have to spend a fortune, an A3 or an A5 will do. All you really need are the numbers and an average heart rate, and perhaps your total exercise time. Even a Polar A1 will give you this info.
In order to make the HR monitor work for you, you need to determine your exercise zones--You can do this in a number of ways.
The simplest way to determine your exercise zones, your levels of intensity, is by using a formula. The Karvonnen or HR reserve method works well.
The Karvonnen or HR reserve method:
You need an accurate average resting heart rate. For a couple of days before you get up in the morning, take your pulse. Count for ten seconds and multiply by six. Or just place your HR monitor on and lay quietly for a few minutes.
This is your average resting HR.
Then take your max--220 minus your age.
220 minus age and minus resting heart rate and multiply by your zones 65%, 75%, 85%
Then add your resting back to the numbers you get.
These are your zones. Zone 2 is exercising at less than 75% intensity.
Zone 3 is exercising between 75 and 85%
Zone 4 is above 85%
NOTE--that these zones aren't hard and fast--some would place Zone 1 or Zone 3 at slightly different percentages. BUT all would agree that above 80%, you are working hard. Above 85% is "redlining" top-end stuff.
Many coaches now use Anaerobic threshold, not maximum heart rate to determine exercise intensities. Anaerobic threshold, or AT, is kind of a magical number for an endurance athlete. It is that place where your body goes from having enough oxygen to work for quite awhile (properly hydrated and fed) to being much more limited in its capacity to continue at that level of intensity.
Anaerobic threshold, once crossed, means you'll soon have to slow down. You breathing is real REAL labored, your muscles burn, and you simply must slow down a bit. You've gone into oxygen debt, and you'll have to pay it back fairly soon.
Lots of good reasons to know your Anaerobic threshold (and it will be slightly different in each of your three sports). I won't elaborate upon all of them now, since this email is already getting long!!
On the bike, here's how you'd determine AT:
Go down to the Autobahn, to
Right before you take off, start your monitor. Then rider around that 8 mile loop just as fast as your legs and lungs will take you. Don't worry about the numbers, just ride hard.
As soon as you come all the way back around, stop your monitor and look for your average HR. (Obviously, you need a HR monitor that will give you this average).
That average HR is a good indicator of AT, anaerobic threshold on the bike. Note, though, that your threshold swimming will be quite a bit lower, since you're prone and you're using less muscle.
Training zones based on AT are quite easy, really. Here's how I slice it. But, if you want more detail, go to Joel Friel's book, the Triathlete's Training Bible.
Easy, recovery days stay 20 beats below threshold
-Long slow endurace and days, and the BULK of your training time are 10-15 beats below threshold.
-Tempo work is 5-10 beats below threshold. There are your long, interval days, intervals may last as much as 10 minutes in well-trained athletes. A harder workout.
-Interval work, real hard, red-line, top end stuff is 5-10 beats above threshold. These are shorter intervals, 2-4 minutes, with full recovery between intervals.
If you don't have a HR monitor and have no plans to get one, don't despair. A good old fashioned talk test can help a great deal.
Basically, for most of your workouts, except the super hard ones, you should be able to talk aloud fairly easily the whole time. This ensures that you are well below threshold.
Only for your hard days do you want intervals, periods of time, where you can't talk aloud easily. When you're breathless, you are likely close to anaerobic threshold.
If you absolutely CANNOT talk, you are anaerobic, which is tough on the body, and which is not much to your purpose as a triathlete.
Unless you want to be a 200 meter track sprinter, you should consider backing off a bit. It's your aerobic capacity that matters the most for what you are attempting to accomplish.