Varicose veins are twisted and enlarged veins, usually found on legs. Excessive pressure on the veins, especially when standing and sitting, is the main reason for the enlargement of these superficial veins, but heredity also seems to have a major role.
Since legs are the farthest extremity from the heart, it requires extra effort to pump the blood up the veins in the leg and back into the heart against gravity. This is done by the calf muscles.
There are paired valves in these veins to prevent backflow of the blood, but as the veins lose their elasticity and become dilated, these valves fail. This results in blood getting pooled in the veins, causing them to twist and bulge out.