Yoga means union. Etymologically, it is connected to the English word, yoke. Yoga means union with God, or, union of the little, ego-self with the divine Self, the infinite Spirit.
Most people in the West, and also many in India, confuse yoga with Hatha Yoga, the system of bodily postures. But yoga is primarily a spiritual discipline.
Not that there's anything wrong with practicing Hatha Yoga. The body is a part of our human nature, and must be kept fit so it doesn't obstruct our spiritual efforts. However, those who are focused on self-realization do not necessarily have to practice it as much or at all.
Hatha Yoga is the physical branch of Raja Yoga, the true science of yoga. Raja Yoga is a system of meditation techniques that help to harmonize human consciousness with the divine consciousness.
Yoga is an art as well as a science. It is a science, because it offers practical methods for controlling body and mind, thereby making deep meditation possible. And it is an art, for unless it is practiced intuitively and sensitively it will yield only superficial results.
Yoga is not a system of beliefs. It takes into account the influence on each other of body and mind, and brings them into mutual harmony. So often, for instance, the mind cannot concentrate simply because of tension or illness in the body, which prevent the energy from flowing to the brain. So often, too, the energy in the body is weakened because the will is dispirited, or paralyzed by harmful emotions.
Yoga works primarily with the energy in the body, through the science of pranayama, or energy-control. Prana means also 'breath.' Yoga teaches how, through breath-control, to still the mind and attain higher states of awareness.
The higher teachings of yoga take one beyond techniques, and show the yogi, or yoga practitioner, how to direct his energy in such a way as not only to harmonize human with divine consciousness, but to merge his consciousness in the Infinite.
The ordinary person's energy is locked in his body. The lack of availability of that energy to his will prevents him from loving the Lord one-pointedly with any of the three other aspects of his nature: heart, mind, or soul. Only when the energy can be withdrawn from the body and directed upward in deep meditation is true inner communion possible.
Yoga is a very ancient science; it is thousands of years old. The perceptions derived from its practice form the backbone of the greatness of India, which for centuries has been legendary. The truths espoused in the yoga teachings, however, are not limited to India, nor to those who consciously practice yoga techniques. All religions also have discovered aspects of the spiritual path that are intrinsic to the teachings of yoga.