Here is a Naruto site (login?) on hand seals:
Hand Seal Guide: Hand seals are as important as Chakra in Naruto. All GenJutsu and NinJutsu need hand seals to be performed, but TaiJutsu doesn’t because it simply needs no Chakra. The main function of a Hand Seal is similar to that of magic words. Instead of speaking of words to activate a spell, Hand Seals are used to activate the technique, and release all the gathered Chakra. Hand Seals stand out for animals, specifically the animals found in the Oriental Zodiac signs. That’s why there are 12 seals, one for each of the 12 years found in Oriental zodiacs… if the hand seals in a NinJutsu are not performed correctly, the Chakra will be released wrong and a different effect will occur, probably failing the Jutsu. As a result, Hand Seals need a lot of concentration, which is why techniques in Naruto cannot be interrupted.
I guess these hand seals are related culturally to mudras, which also function as spiritual enablers. (Interestingly, in both cases the form is usually a posture without explicit movement.) It is almost like there is a heavenly connection between our hands, our minds and the forces of cosmic nature. I am a mere mortal meddling with divine stuff. Better watch out for false gestures lest I get hit by a fireball.