Cardio burns calories, reduces stress, helps you lose weight, and is good for your lungs and heart, but cardio isn't the only thing your body needs. It's also important to strength train to tone your muscles, increase your metabolism, and increase your muscle mass. Of course you also need to stretch as well to increase flexibility, which will prevent injury.
So a good rule of thumb is to include all three — cardio, strength training, and stretching — in every workout. You can start with 30 minutes of jogging, followed by 20 minutes of dumbbell exercises, and finish with 20 minutes of yoga. Not only will your entire body feel worked, but doing all three every time you exercise will make sure you don't neglect one type of training.