Yoga comes from the sanskrit word ‘Yuj’ meaning union. It is basically the harmonisation of the body, mind and energies which leads to holistic wellbeing. Yoga consists of eight stages initially starting at setting discipline and ethical standards for oneself and attaining physical efficiency of the body through postures and breathing. Then it focuses on withdrawing attention from the outside world to inner dimensions, leading to meditation and blissful state called Samadhi.
Here’s a brilliant video explaining yoga in simple terms.