The superior and inferior vena cava are collectively called the venae cavae. They are the veins that return deoxygenated blood from the body, into the heart. They both empty into the right atrium.
The inferior vena cava (or caudal vena cava in animals) travels up alongside the abdominal aorta with blood from the lower part of the body (see # 11 in the diagram). It is the largest vein in the body. 1
The superior vena cava (or cranial vena cava in animals) is above the heart, and forms from a convergence of the left and right brachiocephalic veins that contain blood from the head and the arms (see # 3 in the diagram).