Varicose Veins and Spider Veins

What are varicose veins and spider veins?
How common are abnormal leg veins?
What causes varicose and spider veins?
Why do varicose and spider veins usually appear in the legs?
Are varicose and spider veins painful or dangerous?
How can I prevent varicose and spider veins?
Should I see a doctor about varicose veins?
How are varicose and spider veins treated?
Can varicose and spider veins return even after treatment?