The easiest way to remember the four muscles of the rotator cuff is the mnemonic "SITS." These letters spell out the first letter of each of the four muscles, which have long and complicated Latin names. The SITS muscles are only a subsection of the muscles of the shoulder, but these four form a "cuff" of muscle and tendon tissue around the shoulder joint, and are heavily involved in the rotation of the arm at the shoulder.
The supraspinatus muscle is unusual in the SITS muscles as it does not rotate the humerus, which is the bone of the upper arm, in the shoulder socket. Instead, it helps to lift the arm out to the side, a movement called "abduction." It is included in the rotator cuff because its tendons meld with the tendons of the muscles that do rotate the arm, and because when a person rotates his arm it is often abducted at the same time. The supraspinatus muscle is attached to the humerus bone in the arm and also to the top of the shoulder blade.
The infraspinatus is the strongest muscle that allows you to laterally rotate your arm. For example, it works when you pull your arm back and out to throw a baseball. It is attached to the humerus and the rear side of the shoulder blade.
The teres minor is a little muscle that lies over the lower end of the back of the shoulderblade and attaches to the humerus. The muscle is below the infraspinatus and often appears to be part of the infraspinatus, although it has its own specific nerve supply. The teres minor helps the infraspinatus to laterally rotate the arm, and also helps to "adduct" the arm, which means it helps bring the arm back towards the body.
The subscapularis muscle roughly does the opposite to the infraspinatus and the teres minor in that it medially rotates the arm. For example, if you throw the end of a scarf over your left shoulder, you are medially rotating your right arm. It also adducts the arm, with help from the teres minor. The subscapularis is a flat, triangular muscle that attaches to the humerus and to the front flat edge of the shoulder blade.