Beginner's question about string's join() method

[Macygasp]

Hi,

Can anybody tell me why and how this is working:

>>> ','.join(str(a) for a in range(0,10))
'0,1,2,3,4,5,6,7,8,9'

I find this a little weird because join takes a sequence as argument;
so, it means that somehow, from the "str(a) ... " expression, a
sequence can be generated.

If I write this:
>>> (str(a) for a in range(0,10))
<generator object at 0x7f62d2e4d758>
it seems i'm getting a generator.

Can anybody explain this to me, please?

Thanks in advance.