I've been puzzled about this as well - so, in wordpress-speak, 'categories' are in fact the same thing as 'tags'? the aspect of this that has been confusing me is the fact that the categories appear to be fixed topics, or, well, categories, that appear on the main site at all times. can we also use tags to tag individual posts with keywords specific to that post, but which we do not want as fixed larger categories? for example, I have a list of categories on my main site that roughly separate the kinds of writing I post - notes, memos, meditations, letters, etc. Perhaps these should in fact be page names. But if one single post was about Ingrid de Kok or shelling peas, I would want a way to generate these tags for that one post without adding to the list on the front page.
I hope all this verbage makes sense to someone here....thanks so much all for reading!