What do the shoulder muscles do?

The deltoid caps the shoulder and has three parts. The front part lifts your arm forward, the middle part lifts it out to the side, and the rear part pulls it back. Underneath, the small rotator cuff muscles keep the ball of the joint centred in its socket.

How do you train your shoulders?

Overhead presses work the front and middle of the deltoid. Lateral raises isolate the middle. Pulling movements with the arms out wide, like band pull-aparts, train the rear. Most people already press more than they pull, so it is worth giving the rear of the shoulder as much attention as the front.