Bikram Yoga is the "Original" Hot Yoga.
A lot of studios have incorporated heat into their practice. And that's great. It makes you work, it makes you sweat. But the addition of "heat" does not make it "Hot Yoga".
Bikram Hot Yoga, is a precise sequence of 26 poses and two additional breathing exercises that is specifically designed to work every part of your body and all internal organs.
This sequence is derived from the Classic 84 Asana Ghosh Lineage yoga and is literally a prescription for retaining and maintaining your health and well being. (See "Why Bikram Yoga")
Bikram Choudhury, from childhood, spent his life under the tuteledge of yoga guru Bishnu Ghosh (younger brother of the famous yogi and guru Paramahansa Yogananda) learning his Master's form of hatha yoga. As a teenager, he won the All-India Yoga Asana Championship three years in a row.
As a young man, his master sent him to work in community hospitals using yoga as a method of healing. Though this work, he developed what he termed his "Beginner's Yoga" sequence for health and well being. This evolved into what is known today as Bikram Hot Yoga.
In the early 1970's, at his Master's behest, Bikram brought the Ghosh Lineage Yoga method to the United States. His "Beginner's Yoga Class" became very successful in the Los Angeles area and he became what many would call the yoga guru to the "Stars".
Due to its uniqueness, effectiveness, and popularity, Bikram Hot yoga has spread world wide -- even here to Twinsburg, Ohio.
For more information on the history of this practice read Bikram's book