The word yoga means "union", referring to combining your mind with the physical body. You are practicing it to calm your mind, by holding a present focus and release any limitations in the body.
Yoga can be practiced in different ways, ranging from a physical flow yoga with music to a sedentary breathing exercise in silence. An important factor in practice is the breathing, which allows the body to heal by a more energetic respiration. It's said, according to Patanjali in the Yoga Sutras, the eightfold path are eight steps that basically act as guidelines on how to live a meaningful and purposeful life.
Yama - Moral codes (5)
Ahimsa - non-violence
Satya - truthfulness
Asteya - non-stealing
Bramashayra - sexual restraint
Aparigraha - non-possessiveness
Niyama - Personal behavior (5)
Saucha - purity
Santocha - contentment
Tapas - dicipline
Svadyaya - self-study / spiritual studies
Ishvara Pranidanha - surrender to universe / god
Asana - Yoga postures, it referred to mastering the body to sit still for meditation
Pranayama - Breahting techniques
Pratyahara - Withdrawal of the senses
Dharana - Concentration
Dhyana - Practive of meditation
Samadhi - Merging with the divine