In the Introduction to Yoga, you will realize that yoga has been studied, adapted, and practiced for centuries. Always will it evolve with the masters that have dedicated their minds to yoga and those who have learned from them. As yoga has grown, naturally has the practicer evolved.
This source is an complete introduction to yoga from any point in your yoga practice. You can learn an array of information from yoga exercise and yoga poses to the styles of yoga and how to meditate. And your yoga practice will continue to grow.