First off, deep vein thrombosis (DVT) refers to clots that form in the veins of the legs. DVTs are often the cause of the pulmonary embolisms. Normally, the blood in the veins is pumped back to the heart by the leg muscles that 'milk' the veins through contraction. However, clot formation occurs when a person is on bed rest or is inactive (like mentioned above - long car trips or long hours at a desk). In the hospital a lot of attention is paid to preventing DVTs; patients that are in the hospital post surgery or are on bed rest for more than 24 hours are automatically given heparin, an anticoagulant, that prevents clotting.
These clots when they do happen can break off from where they have built up within the leg veins. The clots then travel to the heart and get pumped out to the lungs - getting lodged and causing a pulmonary embolism.
Again, pictures are worth much more than my words so here is a quick video that illustrates the phenomena of DVTs and PEs:
It's also worth noting, this can occur in any age group. In March 2011, the athlete Serena Williams had a foot surgery in New York and then traveled to Los Angeles, where shortly afterward she had to be rushed to the hospital for a PE (Serena William Hospitalized for Pulmonary Embolism).
So remember to get up and move around to keep your legs and lungs lively!
