Last week a friend asked me some questions about using categories vs. tags in a blog. It seems one of her co-workers is setting up a blog for their software company and wondered which way to go. I suggested they hire me as a blog consultant, but after that was laughed at I helped her out anyway 
Here’s what I think I told her:
Generally categories are going to be broader than tags. A category is a high-level classification of a post, with many posts only really falling into one (or maybe two) categories. People who browse blogs will often view category pages to see posts on a particular general area of interest.
Tags are (or at least should be) specific. A post should have several tags, covering all topics for which it might be relevant. If your blog software has a plugin or feature for decent tag management you can do a lot of powerful things with tags. I use Ultimate Tag Warrior on WordPress and while it’s not very simple to configure, once you get things set up there is a lot of flexibility for tagging. if you’re going to use tags, make sure that you’re setting them up so that Technorati will index them as such. Technorati is one of the top referrers to my blogs, and if you’re going to tag posts, you might as well get some traffic from it.
Should you use categories or use tags? It depends on your purpose, audience, and the breadth of information being shared on the blog. What you’ll see on the blogs I run is a few general categories, and hundreds of specific tags.
