Yoga is an old discipline of Hindu philosophy. It is both spiritual and physical. Yoga uses breathing techniques, exercise and meditation. It helps to improve health and happiness. Yoga is the Sanskrit word for union. Patanjali was a pioneer of classical yoga. He defined yoga as "the cessation of the modification of the mind" (stopping changing the mind).
A person doing yoga will move from one posture (called asana) to another, but there are also other aspects which are important in yoga, such as breath-work, mantras, mudras (postures of the hands and fingers), meditation and much more (for more information, check out the Yoga Sutras). For example, the "sun-solution" contains 12 poses of asanas, one after the other, and is said to help balance body and soul. There is a specific mantra for each asana. The "sun-salutation" is popularly known as "Suryanamaska".