What's the difference between str and 'hello' ?

[hong Yu]

I am new to python.  
I was confused:

>>> a = list('hello')

>>> a
['h', 'e', 'l', 'l', 'o']

I want to combine the items in the list into a string.
So:

>>> ''.join(a)
'hello'

but:

>>> str.join(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: descriptor 'join' requires a 'str' object but received a 'list'

So why TypeError raised? 
and 
What's the difference between str and 'hello' ?

Thanks a lot.