In the Western World, yoga has become a system of physical exercise for better health. Many cities and towns offer Yoga classes for people to take part in. The classes can be for beginners, intermediate or more advanced.
Yoga has a long history as more than just an exercise in the world, though. It's origin dates back to ancient India. There are some seals that were found at sites of the Indus Valley civilizations that appear to have figures of people in different Yoga positions. These date back to about 3,000 years BC. Showing that at least some form of a discipline in Yoga, whether it was called that then or not, had begun.
True Yoga is a form of physical, mental and spiritual practices that are a part of Hinduism, Buddhism, Jainism and Sikhism. There is a goal to these disciplines and depending on which tradition you follow that goal may differ slightly. To put it in a novice term, a connection to one's inner self through meditation is trying to be achieved or a connection to a spiritual level that we are not aware of during full consciousness.
Yoga sutras concentrate on controlling the mind while Hatha yoga deals with the health of the body and its purity.