Pulmonary embolism is a general term that refers to material that travels to the lungs from somewhere else.
Most emboli are blood clots, but some are composed of fat, amniotic fluid, or air.
They travel to the lungs in the veins, which get larger and larger as they approach the heart. Once in the pulmonary arteries, the embolus travels until the blood vessel is too small for the material to pass. If the embolus is small and makes it to a small vessel, there will probably be no symptoms. On the other other hand, if there are lots and lots of small emboli they can cause a significant problem. So can one or two really big emboli.