A hash is algorithm used to check the validity of data. If you hash a file and change one bit the hash comes out differant. If you want more information I'd suggest "Applied Cryptography" and I have a few links.


http://www.anujseth.com/crypto/ (not all the information is up yet)

http://www.itl.nist.gov/fipspubs/fip180-1.htm (This one has information on SHA1 (Secure Hash Algorithm, I believe it's used by gnucleus and others).

You can search for other hash alogrithms like MD5, RC4 etc.