Flexibility: Stretching your tight body in new ways will help it to become more flexible, bringing greater range of motion to muscles and joints. Over time, you can expect to gain flexibility in your hamstrings, back, shoulders, and hips.
Strength: Many yoga poses require you to support the weight of your own body in new ways, including balancing on one leg (such as in Tree Pose) or supporting yourself with your arms (such as in Downward Facing Dog). Some exercises require you to move slowly in and out of poses, which also increases strength.
Muscle tone: As a by-product of getting stronger, you can expect to see increased muscle tone. Yoga helps shape long, lean muscles.
Pain Prevention: Increased flexibility and strength can help prevent the causes of some types of back pain. Many people who suffer from back pain spend a lot of time sitting at a computer or driving a car. That can cause tightness and spinal compression, which you can begin to address with Yoga. Yoga also improves your alignment, both in and out of class, which helps prevent many other injuries.