Cardio is a term that comes from the word cardiovascular, which is the system made up of the heart, veins, and blood vessels that run through a person's body. So, when you do aerobic exercise, you are doing an exercise that strengthens your cardiovascular system. Aerobic exercises are generally low intensity workouts, much lower than strength training for example, but go on for much longer periods of time. An example of aerobic exercises would be, but not limited to; swimming, running, jumping rope, cycling, skating and walking. There are a large number of benefits that having a strong cardiovascular system can bring, some of which are, being less at risk of heart disease, you will have less stress in your life and it can alleviate depression. But one of the main reasons to do aerobic exercise is the fat loss it can bring. Aerobic exercises generally go on for long periods of time and in that time you are bound to burn few calories. When you burn more calories than you take in, your body will turn to its reserves, which are fat. Your body will use that fat as a source of energy, and that's how fat loss happens. Cardio often goes hand in hand with weight training, because without cardio you won't be able to show off what you've gained during your weigh-in.