Reticular veins are formed by larger veins that are located deeper in the skin's surface. They are usually blue or purple in color, and frequently feed areas of spider veins which are closer to the surface. These veins are usually about 2 mm in diameter, and often don't protrude on the skin surface. Reticular veins may form areas of unattractive clusters of veins predominantly located on the inner part of thighs, backs of legs and ankles. They can also be associated with symptoms of pain and discomfort of the legs.
Like spider veins, reticular veins don't exist for no reason. Sometimes (though not always), there is underlying venous insufficiency which caused them to occur.