I've heard that Mr. Iyengar has said that what he teaches is Hatha yoga and the term "Iyengar Yoga" is something invented by his students. That being said, I think there are quite a few unique things in the style of hatha yoga that we call Iyengar yoga based on some core knoweldge, focus & principles that Mr. Iyengar teaches. You'll probably find a better explaination of exactly what those differences are on the websites or in the descriptions of the teacher training programs at official BKS Iyengar Yoga schools. I think it's quite common to experience yoga classes where yoga teachers combine principles and techniques that they've picked up from different souces. So, that can make it difficult to sort out what style they're teaching. This is also compounded by the tendancy among new yoga schools and teachers to look for ways to claim a unique marketing niche by finding a new name for their class (i.e. urban flow yoga, iyengar flow yoga, purusa yoga, urban rhythms yoga, etc.) This doesn't necessarily mean that all these hybrids are bad, but it can be confusing if people don't fully understand the core principles they're borrowing before they innovate and/or change them. Bottom line: to get a sense of what distinguishes Iyengar yoga from other forms, you need to take a few classes from a few teachers who trained & are certified to teach in that style.