For each of these first few sections (cardio, weight training, flexibility) we first start talking about benefits or why someone should choose to include that type of exercise into a weekly routine.
To start off, we need to define several terms related to weight training: http://www.livestrong.com/article/115549-define-strength-power-muscular-endurance/
For many young adults, they get the idea that they really should try to incorporate weight training (you can also call it resistance training or strength training).
The list of benefits for weight training is not as extensive as the benefits for Cardiorespiratory exercise. And these benefits go beyond just hypertrophy (glossary) of muscles. Notwithstanding, weight training does add benefits that Cardio workouts can not:
https://www.livestrong.com/slideshow/1008208-13-benefits-weightlifting-one-tells/
http://www.everydayhealth.com/fitness/add-strength-training-to-your-workout.aspx