The primary muscle of the shoulder is the deltoid (also know as the “delts”). It is the rounded cap over the point of your shoulder. There are three heads of the deltoid that insert into the upper arm bone. The anterior (front) head, the medial (side) head, and the posterior (rear) head.
Around the shoulder joint itself is a small bundle of muscles called the rotator cuff. These smaller muscles help to rotate the arm in its shoulder socket. Since these muscles are small relatively weak they can be injured easily. That is why it is essential to warm up well before working your shoulders or doing any activities that incorporate the shoulder joint.