Simple Caesar shift and deshift
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, '!'