Deep vein thrombosis is a condition where a clot is formed in the deep veins of usually the leg. Such a clot could obstruct blood flow in the leg. A part of the clot may break off and obstruct the flow of blood to the lungs. Here is an animation to help you understand how a clot is formed in the deep veins and the complications of deep vein thrombosis.
this site so cute but if one can download the video it will be better