The pectoralis (pecs) is a fan shaped, thick muscle of the chest.. It is located deep to (behind) the breast.
Primarily horizontal, it originates at the sternum (chest bone), the clavicular junction (where the sternum meets the clavical) and inserts into the humerous (upper arm bone). Vertically it attaches to the clavical (collar bone), to the cartilage of the ribs and the fascia of the upper obliques (stomach muscle).
It's major function is to internally rotate, extend and adduct the humerous.
The pectoralis can increase it's size at a greater rate than other muscles because of their shape and thickness. They are a major contributor in pulling the shoulders forward, rounding them, if left unchecked. To avoid this, the upper back muscles must be strengthened.