Deep vein thrombosis (DVT) is a condition in which a blood clot forms in one of your deep veins, usually in your leg. DVT can cause pain and swelling and may lead to complications such as pulmonary embolism.
DVT happens when a blood clot forms in a deep vein. DVT is most common in the deep veins of your lower leg (calf), and can spread up to the veins in your thigh. DVT can also first develop in the deep veins in your thigh and, more rarely, in other deep veins, such as the ones in your arm.
Deep veins pass through the centre of your leg and are surrounded by a layer of muscle (see diagram). When blood clots form in the superficial veins, which lie just under your skin, the condition is known as superficial thrombophlebitis. These superficial blood clots are different to DVT and are much less serious.
Source - Bupa Website (Click Here)