There are two main problems to consider: reflux and/or obstruction.
Reflux
Reflux is when valve malfunction allows backflow in veins. The pump function is ineffective. Veins stay full, valves don't close, and pressure in the veins remains abnormally high (venous hypertension). If perforating veins develop valve malfunction, the high pressures developed in the deep veins with muscle contractions (up to 250 mmHg) are transmitted to the superficial veins.
Valve malfunction can be Congenital (when a person is born with it), which is rare; or acquired. Valve malfunction can be acquired different ways:
Weak vein walls or valves can give way under "normal" stresses.
High pressures or prolonged pressure (e.g. people who stand in one place without moving for prolonged periods) can stretch vein wails so valves don't work.
Valves can become damaged - for example following DVT.
Venous hypertension can (and usually does) lead to:
Dilated veins - varicose veins and Telangiectasia (spider veins).
Edema - tissue swelling from excess fluid. High vein pressures lead to high capillary pressure and water is actually pushed out through capillary walls.
Skin damage - high vein pressures, and stress on vein walls lead to red and white blood cells sticking to walls and being forced or migrating out of the blood vessels into the tissues. As red blood cells break down their iron is deposited in the tissues leading to a brown discoloration. As activated white blood cells enter the tissues they release chemicals that actually begin damaging the tissues. This may lead to self digestion of the tissues or build up of products within the tissues (e.g. fibrin) that impede diffusion of oxygen and other nutrients. The result of one or both can be death of the tissues surrounding the veins leading to a venous ulcer (sometimes called stasis ulcers or venous stasis ulcers). The typical venous ulcer occurs on the lower leg (usually near the ankle where vein pressures are highest) and is surrounded by skin with a rusty brown color.
Obstruction
Obstruction is virtually always the result of a blood clot (a thrombus) forming within the vein. This is usually secondary to vein wall damage; slow flow (e.g. prolonged bed rest or inactivity) or increased tendency for blood to clot (often seen in cancer patients).
Thrombus in a superficial vein is usually called superficial thrombophlebitis. In deep veins it is called a Deep Venous Thrombosis or often just DVT. This is much more serious than superficial disease because the clots tend to be larger and may break off, travel through the heart and get trapped in the lung (a pulmonary embolus) which impedes blood flow through the lungs. DVT is a very common condition with over a half million occurring in the U.S. yearly. About 10% become pulmonary emboli and 10% of these are fatal.
Most thrombi dissolve spontaneously though slowly (over weeks to months), but about 20% become chronic (i.e. a permanent obstruction). In the majority of cases clots tend to damage valves in the affected segments.