You can easily use your heart rate to set the right intensity during your workouts. Doing this will give you immediate feedback if you are training intensity is aligned with the training goal.
Please note that the following information will vary from individual to individual and offers only approximations. Over my years of training however, I found them to be accurate enough and simple to calculate so I am happy with them.
Since every one will have different heart rates according to many criteria like age, gender, training level and others, a percentage of maximum heart gives us a good reference point.
To find your max heart, use this equation:
Subtract your age form 220
This will give you the maximum number of beats your heart can do within a minute.
There are 5 typical training zones
Zone 1: 50-60% max heart rate
This is the intensity you want to be at for warm up and cool down. Feels very easy.
Zone 2: 60-70% max heart rate
The intensity here is comparable to going on a jog. You are starting to improve your aerobic threshold. This is the threshold at which lactate starts to accumulate in your muscles.
Good for long (30 min+ runs)
Zone 3: 70-80% max heart rate
Things are starting to get rougher here. You are breathing faster and are improving your body's ability to deliver oxygen to your muscles and remove carbon dioxide.
Good for tempo runs and longer intervals (work intervals longer than 3 min)
Zone 4: 80-90% max heart rate
You are breathing hard and your muscles get tired quick. You reach your anaerobic threshold at this intensity. This is when your body can't remove the lactate as fast as it produces it. This is what really tires out the muscles.
This is the zone for most interval training with interval between 30 sec and 3 min.
Zone 5: 90-100%
Cannot be sustained for much longer that 15 seconds. You are going all out and working on power.
Good zone for very intense interval training like Tabatas.
Please note there is not "fat burning zone". Each zone will use different fuel sources and impact fat loss differently.
Do I need a heart rate monitor?
Unless you are training seriously for an endurance event I would not bother wth it. The easy way of measuring your heart rate is to count your pulse for 15 seconds and then times it by 4. This will give you the amount of heart beat per minute which you can then use to compare to the percentage of max heart rate you are targeting.
Using this method gives you a solid reference point for your training intensity. This will help you work out better and reach your goals faster and safer.