Yoga is an age-old practice that brings physical, mental and emotional well-being. In fact, thanks to yoga we can counteract back pain, joint pain, hormonal imbalances, headaches, digestive and sleep troubles and so on. We can better manage stress and emotional load. The quality of our breathing improves, a feeling of existential balance is established and the capacity for mental concentration increases.
These are generally the reasons that induce people to approach this wonderful world, and they are all more than good reasons to start. It should be made clear, however, that though true, these are somewhat secondary effects, and not the very heart of Yoga.
In Sanskrit Yoga means union, integration, synthesis. A synthesis of mind and body, of the world and the I and of all the oppositions that characterize the way we tend to conceive and experience every aspect of our life.
In fact, what allows us to become skilled and capable of successfully engaging in life, the fascinating process of growing up and developing, is also a path that goes through a progressive split from the world. By learning to select, analyze and define the world with words and concepts, we lose it at the same time, that is, we lose direct contact with reality. This is why childhood memories have that particular flavor and wholeness, which we hardly encounter in our adult experiences. The child is so bound to the world that when he eats an apple, he becomes one with the apple, when he runs on the lawn he becomes "pure running", without mediation or any kind of  interference, that is, without experience itself as a separate entity. It would be nice to regain this first-hand experience on reality, without however descarding the background of experiences and knowledge that have allowed us to grow up.
So, we could define yoga as a way to open up to the present moment, in which oppositions, distances, dualisms, typical of our mental structures, are solved into a fullness of being.