Spider Veins Vs Varicose Veins

There are a several key differences between varicose and spider veins.

Spider veins are smaller, usually less than 1 mm in diameter. Varicose veins are larger.
Spider veins are superficial in nature (surface), whereas varicose veins usually involve the major veins of the legs.
Spider veins are flat, varicose veins (when at the surface) are bumpy or bubbly looking.
Varicose veins do not function properly. Blood pools in the legs because the valves inside the varicose veins do not prevent backflow (blood moving in the wrong direction).

That's a great question.  There are several differences that I have summarized below:

Spider veins:  usually much smaller in size, up to 1 mm in size.  They are typically purple to purple-red in color and typically asymptomatic.  They are best treated with sclerotherapy, very rarely is laser therapy indicated.

Varicose veins:  these are larger, blue, bulging veins, usually several mm in diameter.  They can be symptomatic and cause a feeling of leg "heaviness", pain, itching, swelling, and even pigmentation of the skin if long standing vein disease is present.  These can be addressed with foam sclerotherapy, but ultrasound evaluation of the venous system is usually advised prior to treatment.  If varicose veins are related to larger tributary veins in the leg, closure of larger veins using endovenous laser ablation may be necessary.

A telangiectasia is a confluence of dilated intradermal venules of less than 1 mm in caliber. Telangiectasia are more commonly known as spider veins and thread veins. They can be pink, red and have different hues in the red to purple blue range. Spider veins occur in 15 % of men and 25 % of women in the general population.In the classification of veins, telangiectasias are classified as type I veins. Spider veins are amenable to laser therapy and sclerotherapy.
A varicose vein is a subcutaneous dilated vein larger than 3 mm in diameter in the upright posture. Varicose veins are also known as varix, varices and varicosities. Varicose veins can occur in ten quadrants: anterior thigh, medial thigh, lateral thigh, posterior thigh, anterior leg, medial leg, lateral leg and posterior leg. They can also occur in the medial and lateral ankle. Varicose veins pool blood in stagnant segments and get inflamed causing chronic phlebitis. If these symptoms of aching, pain, itching, burning and cramping are not relieved with use of compression stockings, they are amenable to the latest techniques employed by many plastic surgeons.