It is a question of definition whether the buttocks are part of the torso or part of the legs. Likewise, the neck and the shoulders can be seen as parts of the torso or not.
The torso in art
Some drawing teachers such as Burne Hogarth teach their students to begin any figure with its torso and then add the other parts to it. The reason for this is, whenever the drawing of the torso is modified, all parts that are attached to it will have to move with it. But the head, arms, or legs (when not bearing any weight) can be drawn and modified freely without affecting the rest of the figure.
The important bones of the torso are the backbone, the ribcage, the shoulder bones, and the pelvis. The main volumes are the ribcage and the hip/abdominal volume. The flexible backbone that connects these two volumes allows the torso to bend and twist.