Yoga is one of the six darshanas (or schools) of ancient vedic philosophy. Yoga concerns itself with the union of the individual self with the infinite universal self. For a fuller discussion of the six darshanas see hinduism. There are many paths within yoga but because of the popularily of physical yoga in the West, the term is often used to mean only the physical practices, which are more correctly referred to as asana or yogasana.
It is not a religion, although some people use yoga to achieve spiritual enlightenment. It is not calisthenics, although it does exercise, stretch, and strengthen the body. Yoga is a system of exercises designed to improve the body's physical health and clear the mind.
Many different types of yoga exist, each with its own philosophies and practices. Some yogas are meditative and focused on spiritual centeredness, while others are more physical and are based on poses, or exercises, called asanas. The history of yoga goes back at least five thousand years. Yogic philosophy was codified around 200 AD by Patanjali in the Yoga Sutra.
The most common type of yoga taught in the West is hatha yoga. Hatha, a Sanskrit word that means "sun" (ha) and "moon" (tha), represents the opposing energies--hot and cold, male and female, positive and negative, yin and yang. Yoga, translated from yuga, the Sanskrit word for yoke, means union. Hatha yoga attempts to balance the mind and body through physical exercises and controlled breathing.
Some modern schools and styles of Yoga: