The inferior vena cava is formed by the union of the left and right iliac veins, going up retroperitoneally along the posterior wall of the abdominal cavity, anterior to the vertebrae and to the right of the abdominal aorta to finally empty into the right atrium of the heart.
It brings back blood to the heart from everywhere in the body inferior to the diaphragm, whilst the superior vena cava drains everything above the diaphragm.
There are exceptions to this in anastomoses between the two. Two good examples are the azygos vein system and in the venous plexuses next to the spinal cord but the preceding statement is largely true.