Tears or Lacrimal Fluid is produced from the Lacrimal Gland which is located above and lateral to the eyeball. It secretes the fluid onto the eyeball via Lacrimal Ducts which are up behind the eyelid. When you blink the eyelids push the fluid over the eye and medialy (ie towards the nose). It then sits a the Lacrial Lake which is where you see the water build up when crying. The water is absorbed from here through the Canaliculi and into the Lacrimal Sac from where it travels into the nose through the Nasolacrimal Duct (this is why your nose runs if you cry).
Unfortunately when excess fluid is produced the lacrimal lake does not have a high enough capacity to hold it all until its removed so some fluid overflows and runns down the face. The function of the fluid is to, by continusouly flowing away into the nose, catch and remove any small particles from the sirface of the eye and thus helps keep vision clear.
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