To some people Ashtanga is the physical origin of vinyasa flow, but to others it is the physical limb of Raja Yoga. There is truth to both sides of the story, but I will briefly try to answer the question: "What is Ashtanga?"
Thanks to the work of Sri K. Pattabhi Jois, students across the globe are able to practice the same sequence of movements, no matter where they study and under what teacher they study under. He standardized yoga teacher training requirements within his style. This consistency is perfect for instructors and students who enjoy a bit of structure in their practice.