Hash questions

Hi!
Nice if someone could give me a minute to explain som hash basics for me.

Is hashing a technique that tries to make an unique value out of a file? Can I calculate the average hex value of the bytes in a file and call it the files hash value?

Does Metadata screw up the hash?

Can Evil Forces fool the hash system? Like, can I make a (copy of a) file that has the same name, filesize and hash as the original, in order to f*** with those who want to download the file?

Kaya_man, do you smoke hash?

Thanks!

Stig Eide, (stigeide@yahoo.com)