Phonologies

David Peterson mentioned in his explanation of his first conlang that:
           1.) Though it had a large "phonology", it wasn't a phonology
at all: It was just a list of sounds.

Being rather new at this, I don't understand the difference. So, could
someone explain what a phonology *is* and what it's supposed to *do*?

Thanks.
--
=============
James Worlton
          "We know by means of our intelligence
          that what the intelligence does not
          comprehend is more real than what it
          does comprehend."
                           --Simone Weil