I think most Yoga in the west is has been diluted to something closer to calisthenics than to its original form. From my understanding, true yoga is spirituality in action. Reading the sutras of patanjali that Leo mentioned is a good idea to gain an understanding of it. It's the principle text upon which most of modern yoga is formed. It's actually really short and will provide a lot of insight. When most people hear the word, they think of physical movement and contorting your body into certain postures but these are called Asanas and it's just one branch of yoga. In some traditions, asanas are really just a way to limber up the body physically and energetically in anticipation of the more intense meditative practices. Practices can range from meditation to self inquiry, taking action and 'doing your duty' in life (called karma yoga) or even pretty mind-altering breathing exercises.
Here's a good pdf translation of the sutras:
http://www.swamivenkatesananda.org/clientuploads/publications_online/Enlightened_Living_by_Swami_Venkatesananda.pdf