Target Heart Rates
It is recommended by the Surgeon General that you should get at least 30 minutes of exercise on most days. The American Heart Association recommends the same thing, but says you can work out three times per week only if the exercise is vigorous.
Both of these recommendations are clear on the time spent, but unclear on how hard to push yourself. That is what the target heart rate is for. It tells you if you are working hard enough or overdoing it. This is important because overdoing it leads to injuries and not doing enough limits progress.
As a general rule, you can get close to the target heart rate by taking 220 minus your age. The American Heart Association publishes a chart to show you the ranges. http://www.americanheart.org/presenter.jhtml?identifier=4736 You may also wish to try this target heart rate calculator. http://www.stevenscreek.com/goodies/hr.shtml
Targeting Without the Chart
Many people like to talk as they work out. If you can talk comfortably, you may be alright and you don't have to worry about overdoing it. If you are constantly out of breath or must completely stop for a few minutes, you are working too hard. If it seems like you can talk or shout or do other things, you are not working hard enough. Many people will shoot for slightly uncomfortable, but not so much that they can't maintain a pleasant conversation.
Video explanation here.