This might not be quite what you've got in mind, but one big improvement you could make would be to use meaningful variable names and insert whitespace.

    def decrypt():
        cyphertext = raw_input('Give me the word to decrypt:')
        shift = input('What was it shifted by?')

        cypher_chars = list(cyphertext)
        str(b) # this does nothing; you should remove it
        cypher_ords = [ord(x) for x in cypher_list]
        plaintext_ords = []
        for i in cypher_ords:
            plaintext_ords.append(i - shift)

        plaintext_chars = [chr(i) for i in plaintext_ords]
        plaintext = ''.join(plaintext_chars)
        print 'Decryption Successful, your word is', plaintext, '!'

Of course you could actually compress much of this into a one-liner. But for a new programmer, I'd suggest sticking with readable variable names that make it clear what's going on.

Still, you could do that while compressing the code a bit:

    def decrypt():
        cyphertext = raw_input('Give me the word to decrypt:')
        shift = input('What was it shifted by?')

        cypher_ords = [ord(x) for x in cyphertext]
        plaintext_ords = [o - shift for o in cypher_ords]
        plaintext_chars = [chr(i) for i in plaintext_ords]
        plaintext = ''.join(plaintext_chars)
        print 'Decryption Successful, your word is', plaintext, '!'