Here is post from a blogging expert that talks a bit about all this tagging and categorizing nonsense:
A category is a table of contents for your blog posts, segregating your posts with grouped like-content.
A tag is an index word that helps you micro-categorize your blog posts. Tags are also keywords and search terms people use to search for the content and subject matter of your post. They are specific to the post content but can be used repeatedly to micro-categorize your blog post content.
See the post for links that go deeper into the subject…
p.s. Importing photos from facebook to flickr