Anatomy
There are three divisions of the veins in the lower limbs
1. Superficial - the veins that run near the surface,
2. Deep - tend to run within deep compartments and muscles,
3. Communicating (also called perforating) Veins - connect the superficial to the deep veins.
Function
Veins return blood from the tissues to the heart. Veins are very compliant, that is their volume/size varies with pressure. They also contain valves which let blood flow in only one direction. In the superficial and deep veins flow is distal to proximal (i.e. foot end of leg towards heart) and in the communicating veins it is superficial to deep. Veins can return blood to the heart passively or actively.
Passive - veins fill and then simply act as pipes. This is not very efficient, and when patients are standing or sitting the pressure at the "bottom of the pipe" can be high (up to 90 mmHg).
Active - using the compliance of the veins and their valves - squeezing the veins by muscles, even with small movements, propels blood out of the leg. This partially empties the veins, valves close, and pressure at the "bottom of the pipe" is much lower. A normal person whose vein pressure at the ankle is 90 mmHg when standing, will lower the pressure to only 20 mmHg with just a few steps.