The yoga poses or postures are also called asanas, and each of them includes both a mental and physical component. Asanas address almost all the body parts, and most often the instructor will indicate the most intense point of physical stress. For further clarification consider the fact that, depending on the yoga variant practiced, the asanas can differ a lot too, particularly since some postures are specific to one school or another.
Besides the physical capacity to perform yoga poses, one also needs to develop balance and an ability to concentrate, since physicality is just one side of the practice, and is completed by the spiritual dimension. Through learning breathing and relaxation techniques you'll come to ignore the difficulty of a posture as well as any distractions in the background. The idea is not only to achieve a certain pose but also to hold it for a certain period of time.
You can find information on how to perform yoga in books, on internet sites, e-guides and other information materials. However, the practice of yoga poses without professional guidance and assistance could turn into a threat to the practitioner's physical and spiritual health with the risk of getting injuries. Therefore, do not start yoga practice by word of mouth, or following directions in books, on the internet or through videos. Beginners have to be under the direct guidance of a trainer, with the understanding that the beginner stage can often extend way beyond a year of practice. The complexity of the yoga poses will increase with every advance you make on the way, but progress remains slow on average as one learns to push the limits of body and mind.
