PyCrypto - How does the Initialization Vector work?

I'm trying to understand how PyCrypto works to use in a project but I'm not fully understanding the significance of the Initialization Vector (IV). I've found that I can use the wrong IV when decoding a string and I still seem to get the message back except for the first 16 bytes (the block size). Am simply using it wrong or not understanding something?

Here's a sample code to demonstrate:

import Crypto
import Crypto.Random
from Crypto.Cipher import AES

def pad_data(data):
    if len(data) % 16 == 0:
        return data
    databytes = bytearray(data)
    padding_required = 15 - (len(databytes) % 16)
    databytes.extend(b'\x80')
    databytes.extend(b'\x00' * padding_required)
    return bytes(databytes)

def unpad_data(data):
    if not data:
        return data

    data = data.rstrip(b'\x00')
    if data[-1] == 128: # b'\x80'[0]:
        return data[:-1]
    else:
        return data


def generate_aes_key():
    rnd = Crypto.Random.OSRNG.posix.new().read(AES.block_size)
    return rnd

def encrypt(key, iv, data):
    aes = AES.new(key, AES.MODE_CBC, iv)
    data = pad_data(data)
    return aes.encrypt(data)

def decrypt(key, iv, data):
    aes = AES.new(key, AES.MODE_CBC, iv)
    data = aes.decrypt(data)
    return unpad_data(data)

def test_crypto ():
    key = generate_aes_key()
    iv = generate_aes_key() # get some random value for IV
    msg = b"This is some super secret message.  Please don't tell anyone about it or I'll have to shoot you."
    code = encrypt(key, iv, msg)

    iv = generate_aes_key() # change the IV to something random

    decoded = decrypt(key, iv, code)

    print(decoded)

if __name__ == '__main__':
    test_crypto()

I'm using Python 3.3.

Output will vary on execution, but I get something like this: b"1^,Kp}Vl\x85\x8426M\xd2b\x1aer secret message. Please don't tell anyone about it or I'll have to shoot you."