The six movements (forward, backward, side, side, twist, twist) are key in a well rounded yoga session. On top of these six movements a well rounded practice should also include a balanced stretch of all the muscles in the body. This allows tension to release, organs to detoxify, strength to be built and flexibility to integrate. A steady flow of breathing through-out your asanas will ensure optimum results of all those benefits.
Back-bends are my favorite part of yoga because they open the heart! Back-bends open the lungs and can help improve breathing, they improve our posture, and they leave us with a sense of openess and lightness.
In Hatha Yoga we focus on finding a balance of sun-moon energy, and it is the back-bends that build heat in the body, create energy and focus on the sun energy. Back-bends give us tapas, a fire-burning energy that motivates and invigorates us.
If you are new to yoga some gentle back-bends include sphinx, cobra, bridge pose, and half-circle pose. Deeper back-bends include upward dog, pigeon pose, camel pose, bow pose, and wheel. Yoga Journal Website has some amazing examples of back-bends, including some videos and other resources.
Always consult with your doctor before beginning any physical fitness routine, and find a certified yoga instructor who can guide you safely through the postures.