It is the outer membrane around the heart that protects it from attacks by exogenous pathogenic factors. The pericardium is capable of being attacked by exogenous Heat, but the Heart is not (directly). In TCM theory and herbology, this is the only way the Pericardium is usually distinguished from the Heart.
In Acupuncture however, the Pericardium has it's own meridian and is just as important as the heart Channel itself. Many points on the Pericardium Channel are used to treat the Heart, and many strongly influence the mental state.
Pericardium influences relationships with other people: Points on its channel are often used to treat emotional problems from relationship issues. Pericardium channel particularly influences the area in the center of the thorax.