Normally, there is a bit of tissue called the sino-atrial node that controls the contractions of the heart. Imagine a pebble dropped in the middle of a still bucket of water and watching the ripples spread towards the edges. Mother nature designed a back-up system in the ventricles, which can also produce beats. A premature ventricular contraction is when both systems work and you end up with an extra beat here and there. Such beats are common in everyone, especially when one drinks coffee or has a coca-cola. They reflect the fact the ventricular tissue is more sensitive. They can also be caused by abnormal levels of potassium or magnesium, as well as lack of oxygen. If there are too many of them your physician may prescribe medications, but just because they have shown up during an examination is nothing to worry about.
Are you talking about PVC's? In the heart it is called Premature Ventricular Contractions, the bottom of your heart contracts an extra beat here and there.