The pulmonary veins carry oxygen-rich blood from the lungs to the heart. There are four such veins, according to About.com, and they all connect to the heart at the left atrium.
The pulmonary veins are an important part of pulmonary circulation. According to WebMD, oxygen-depleted blood is carried by the pulmonary arteries from the right ventricle into the lungs, where it enters a network of capillaries to exchange waste gases and take up oxygen. The fresh blood then flows into the bronchial veins and is taken up by the pulmonary veins for return to the heart.
According to Wikipedia, two pulmonary veins attach to each lung and drain three or four bronchial veins. Along the route back to the heart, the pulmonary veins run inferior to the pulmonary arteries and are fixed to the pericardium. Most people have four pulmonary veins, but in many people the pulmonary veins sometimes merge prior to their return to the heart. As a result, the number of pulmonary veins found in healthy people varies from as few as two to as many as five. This variation is found in a significant fraction of the population and is regarded to be part of normal anatomical variation for humans.