What's the difference between deep vein thrombosis and varicocele?

Big difference: A deep venous thrombosis means that you have a blood clot. These are usually painful, cause swelling in the affected leg, can cause severe long term complications called post thrombotic syndrome, and can result in a pulmonary embolism that can be fatal. A varicocele is basically a varicose vein in the scrotum draining the testicle. Although varicoceles feel funny, they can't possibly kill you.

Difference: A deep vein thrombosis refers to clot within a deep vein of the legs or arms. A varicocele refers to a prominent/dilated/enlarged vein.