Swami Vishnu Devananda has succinctly summazied the key aspects of yoga in 5 simple points. These are:

Proper Exercise - Asanas

Yoga physical exercises are called asanas, a term that means 'steady pose'. The yoga asana is meant to be held for some time. Our bodies are only as young as they are flexible. Yoga postures focus first on the spinal strength and flexibility. Asanas work on the internal organs, muscles and glands.

Proper Breathing - Pranayama

Most of us breathe shallowly and do not use the full lung capacity to breathe. Yoga emphasizes on abdominal and full yogic breathing where in the diaphragm, lung and chest are fully used to draw in more oxygen.

Proper Relaxation - Savasana

Rest and relazation are the natural way to recharge our body. However, most of us even while resting, we expend a lot of energy because of tension. Yoga recommends physical, mental and spiritual relaxation.

Proper Diet

Positive Thinking and Meditation - Vedanta and Dhyana

To achieve the state of lasting happiness and absolute peace, our minds should be calm. Bu focusing the mind on self, we can reach the state of meditation.