It seems to me that the only important difference is that categories can be used in the post url – a post inside the “beer” and “food” categories, by default, will have this url:
/food/beer/2008/09/09/foo-bar.html
Instead of this one:
/2008/09/09/foo-bar.html
See the post tests for details
Tags have no influence in the urls, at least according to their tests.