Yoga is something that is normally associated with relaxation, meditation, or looking inwards spiritually. There are certain styles of yoga, on the other hand, that can be quite intense physically - begging the question, is yoga a good way to lose weight?
This depends largely on the the style of yoga; while some styles which are more about long and deep stretches and relaxing your mind are perhaps not the best way to lose weight, other styles offer a more intense regimen of moves that can be good for both cardiovascular exercise as well as building muscle.
The more relaxed styles of yoga, such as Iyengar or Yin yoga, as they are more about holding stretches or poses for long periods of time, are very good for toning your muscles and for your posture. On the other hand, because they are not intense enough to raise your heart rate very much, these styles of yoga may not be the best for weight loss. Still this doesn't mean that these types of yoga can't contribute to your weight loss program, it just means that they won't be sufficient on their own; they could make a great complement to some other cardio routine, say, jogging 2-3 times a week. Also, if you are very out of shape, one of these more relaxed types of yoga might be a good place to get started, in order to get yourself prepared for the more intense yoga styles.
If you are not very out of shape, on the other hand, and would like to get straight into the more intense yoga, there are a number of yoga styles that constitute a cardio workout of their own. Power yoga, as the name suggests, is a much more physically intense style of yoga, and because it is done in a heated room, you might sweat some of your weight off (some people would say that's cheating, but if it's pounds you are after rather than just your overall health, sweating a lot will help you lose weight - just make sure you don't get dehydrated). Other types of yoga that involve more physical movements are Bikram, Moksha (or any other style of hot yoga), and Ashtanga.
Ashtanga yoga is quite vigorous, and repeats the same series of poses each time so that you can build the muscles required for those specific poses, and get better and better at them each time. Once you learn the poses, you can also do them at home alone, or with a small group of friends. Even in situations where there is a teacher present, students are often allowed to work at their own pace (so you don't need to worry about keeping up with the group if you are a beginner.
Power yoga, which is popular at gyms and health clubs as well as yoga studios, is more intense (and hotter) than Ashtanga yoga, and it does not always repeat the same series of poses, so if you like to do something new every time (and if you want to get the most intense workout you can from yoga) the Power yoga is probably your best bet.
Styles of Hot yoga, like Bikram or Moksha, are also quite intense, and done in a hot room as well. While Bikram, like Ashtanga, follows the same series of poses each time, Moksha, while otherwise very similar to Bikram, has a little more variety to it as far as poses go.
That being said, there are still those who are sceptical about yoga's weight loss benefits compared to other, more traditional forms of cardio or weight training for the sake of losing weight. It is rare to see yoga being toted as some miracle, quick weight-loss trick, like fad diets, or purges or other weird things that are not good for you. So, if your goal to lose weight fast, yoga might not be your best bet. On the other hand, if you have the patience to lose weight gradually, as part of a more generally healthy and stress free lifestyle, than yoga (coupled with a healthy diet and lifestyle) might actually be the very best way for you to lose weight. Losing a lot of weight at once, while it may seem attractive, is not necessarily the best thing for you in the long run - miracle diets tend to not be good for you, and can often just lead to gaining your weight right back again. On the other hand, a generally healthy diet and lifestyle coupled with yoga to improve your posture, tone your muscles, and reduce stress might be the very best way to gradually take weight off, and to keep it off, for good!