Yoga is most likely the most ancient, diverse, scientific, and well rounded form of fitness one can do. Traditional yoga incorporates three components to create a complete practice. Practice that does not include all three components would just be a simple exercise, not yoga.
The three components of traditional yoga include Asana (postures), Pranayama (breath control), and Meditation (concentration). The three work together to develop the physical body, the overall well being of the body's nervous system, and the awareness of mind and self.
Asanas bring health, steadiness and lightness of limb, securing a fine physique which is strong and elastic, without being muscle bound. Pranayama is breath and vital control. The breath is the link between the mind and body. If we can control our respiration we can control every aspect of our being. Meditation is a family of self regulating practices that focus on training attention to bring mental awareness.
Yoga "Yoke", - to bind, to attach, to direct, to attach, to direct. A science that is reached by practice. A powerful method for personal evolution learned through experience.
The world of yoga, with its myriad of styles and approaches is like a forest filled with a variety of color. Every tree in a forest has the same goal, to reach toward the light. When practiced with awareness, the tree grows. Practice is the only method of feeding it. Practice, practice, practice. References: (yoga sutras)(Patanjali)(Coulter)(Iyengar)