Cindy asked about what was more important in terms of blogging — blog categories or blog tags. Infact they are both useful ways of describing how blog posts relate to each other. I would advise against leaving a blog post uncategorized because that really doesn’t let the reader know anything about that post. Additionally, depending on how your permalinks are set up, you may be producing unhelpful links such as http://domainname.com/uncategorized/blog-title.
Blog posts can be viewed chronologically, by search term, by category or by blog tag. What this actually looks like will depend largely on your blog theme or template. Let me demonstrate with an example from this blog.
If you go to the Artworks category of this blog you will see all the posts in that category. (Because of the way my blog theme is set up you will see post excerpts rather than the whole blog posts.) But suppose you were only interested in looking at my acrylic paintings or coloured pencil drawings. Well you can do this just by clicking on the tag links in those entries. I even included tags for the American spelling (colored pencil drawings) to help people searching with this alternate spelling.
How you organise your blog is a very personal thing. You need to do it in ways that make sense to you. But the ability to sort content by category or tags is very powerful. Let me give you an example. Suppose you had a blog about writing, you might have categories for the different stages of the writing process — the initial concept, the first draft, revisions, approaching publishers etc. If you were to include your book title in every relevant post your blog visitors could follow that particular title throughout the whole process. Of course you could do the reverse if that made more sense to you. You could make your book title the category and use tags to describe the stages.
There are related post plugins that recommend other possibly related blog entries based on shared category and tag information. If you intend on using something like this it is worth taking a little bit more time to consider your tags and categories.
I have been referring primarily to the WordPress blogging platform. Other platforms may do things differently. To the best of my knowledge Blogger blogs don’t have categories per se, and rely exclusively on tags (though they call them ‘labels’).