In a brief summary yoga is not about cardiovascular training per se. Cardio in effect is mostly what will help you to lose weight. With that said it will help you to tone your body & improve your overall health.
Yoga will address certain factors which are attributed to weight gain, such as stress and poor nutrition. Stress and poor nutritional choices have adverse physiological consequences.
When you are stressed out you generally do not eat well and many people tend to start binge eating.
Also it is worth while noting that stretching will help with metabolism. Improved metabolism will lead to improved digestive health.
With extensive stretching you will build core strength which will also increase your flexibility and will help to tone your muscles.
The meditative aspects of yoga also have a calming effect on the body. This is achieved with breathing exercises and learning to breath properly has many benefits such as reducing stress.
There are certain types of yoga that are more physical than others. One example of this is Ashtanga Yoga. A general session of Ashtanga is at least an hour and a half if not up to two hours and is both focused and intense.
Due to its intensity more energy is required which can involve a more extensive workout. Because of the intensity of the workout you will tend to burn more calories. All of the above will lead to weight loss but will also have the added benefit of building stability & core strength.
As mentioned previously always consult a health practitioner if you have any questions in relation to any workout regime.