The pulmonary veins are the only veins in the body that carry oxygen-rich (or oxygenated) blood, bringing it from the lungs back to the heart. 

Typically, veins carry deoxygenated blood (blood that has given up oxygen to the body's tissues) back to the heart. The pulmonary veins are an exception to this rule, as they transport oxygenated blood from the lungs to the left atrium of the heart.