As gshoemate indicated, there are many ways to practice yoga, and the manner in which you practice will greatly affect its impact on your weight-loss efforts. Most yoga in the United States can be broken up into two categories: Stretching and Viniyasa.
Stretching will provide many benefits, but significant weight-loss isn't one of them. Rather, it will help improve your other physical activity performance, which in turn may help with weight-loss. I used a combination of this stretching-type of yoga (Kripalu, specifically) two nights per week, and walking three miles a day five days a week, to lose about 50-60 of the 100 pounds I lost in 2001-2.
Viniyasa is a more flowing type of yoga, which qualifies, when practiced with a measure of intensity, as a great aerobic-plus-strength-training work-out. I burned about 7-8 calories per minute doing Viniyasa, and it was a major contributor to losing the rest of the 100 pounds (in Wintertime, I wasn't walking as much), and carried me through non-weight-loss-related improvements in fitness, to the point where I was able to do things like do vigorous all-day hikes in the mountains.
I couldn't imagine starting with Viniyasa. It is tough; I really needed to work up to it. By the same token, for most overweight people, it isn't really sufficient to rely on the gentle stretching type of yoga as one's sole type of exercise: some type of aerobic exercise is necessary, such as walking, to the extent your doctor recommends it.