Posterior is toward the back of the body. For example, your spine is on the posterior side of the body.
The back consists of a variety of layers in the upper and lower portions.
The most superficial muscle of the upper back is the trapezius. This broad muscle extends, flexes, and rotates the head and neck. The levator scapulae attaches the scapula to the upper vertebrae, it elevates the scapula. This muscle tends to get overworked because it can do the job of other muscles. The rhomboids minor and major attach the middle of the scapula to the vertebrae below the levators, they adduct and stabilize the scapula. The supraspinatus, infraspinatus, and teres minor all attach to the scapula and are part of the rotator cuff muscles that move the shoulder.
The deeper muscles support the spine. The erector spinae is a group of three muscles that extend and flex the vertebral column. Even deeper is another group of three muscles that include the semispinalis, multifids, and rotatores. These muscles assist in rotation and extension of the vertebrae.
The lower back is covered by the latissimus dorsi. This muscle wraps around your side to attach to the arm, it extends, adducts, and rotates the shoulder. The quadratus lumborum that attaches to the lower vertebrae and the pelvis, elevates the hip and aids with lateral flexion. The iliopsoas attaches to the lower vertebrae and the leg, it flexes the hip.