The term "posture" refers to the way the body is held (e.g., straight, slumped, etc) and its position when sitting or standing. For singing, it is best to stand rather than sit, as this allows the most open and unrestricted passage of air for both breathing and singing. Of course, if you are accompanying yourself on piano you will have to sit. In this situation, it is essential to sit up straight but relaxed for the best sound.
The spine is not straight and the head and pelvis both tilt forward. In this position, it is not possible to move freely or produce the best sound.
The spine is comfortably straight and in line with the head, legs and pelvis. This position keeps the airways open and makes movement easy and comfortable.