Heart rate is one of the most accurate easily measurable estimates of exertion. Age and gender are the main determinants of max heart rate, the simplified way to estimate your max is to subtract age from 220 if you are male and age from 226 if you are female. There are more complex formulas that involve your resting heart rate as well, but even that is only a prediction, but in my experience it is close enough to get you in the right range. Okay, so once you have determined your maximum heart rate, you can figure out what 60%, 70%, etc. of your max is. Theoretically, if you are working at about 60% you are using fat as your primary fuel source (but don't get too caught up on this, as your overall calorie burn is MUCH more important than the source of your calorie burn). When you are working at 60%-70% of your MHR, you are building your aerobic system, strengthening your heart to some extent, and becoming more fit overall. Higher heart rates of say 70-85% of max burn significantly more calories, you actually increase the size and strength of your heart and lungs, and a lot of other physiological things take place. Simply stated, getting your heart beating in this zone is VERY good for you. Higher than 85% is really uncomfortable, so it cannot be sustained for long, so the benefits are outweighed by the practicality of working at that effort.
Heart rate monitors seem to be most effective for ensuring you are working at a high enough effort while doing cardio or for doing interval training. If you were considering getting one just to wear while weight training, I would sugguest just saving your money because weight training generally doesn't elevate heart rate enough for the monitor to be practical. It can be a great motivator during cardio, though, and some people swear by them, but it all depends on what your goals are and how you plan to use the monitor. Hope that helped a little bit.