What's the difference between tags vs. categories in your blog?  A lot.  Knowing the difference between tags and categories can help you better structure your blog for browsing and SEO.  Personally, I hadn't noticed the issue before because I only have tags on my MSDN blog.  As part of my research on effective blogging practices, I hit the issue.  Now that I've experimented with a few blogging platforms, the difference between tags and categories is more obvious.  For example, WordPress 2.3 supports tags in addition to categories.

Categories, Internal Tags and External Tags

• Categories. Categories are your high-level buckets.  You should be able to chunk up your blog by a small, mutually exclusive set of categories.  Imagine a user trying to browse the broad themes of your blog.  Categories can also become part of your URL.
• Internal tags.  Internal tags are for finer-grained slicing and dicing and hopping across your categories.
• External tags.  External tags, such as Technorati and del.icio.us are for showing your conent in the relevant topics and niches at Technorati and del.icio.us.

Tag Clouds
I think the big benefit of tags is creating browsable tag clouds where you can discover related content.  Whereas categories are just one topic, you can use tags to find related content.  For example, you might browse a "security" tag and then browse a "performance" tag to find the intersection of content tagged both "security" and "performance".

Notes from Lorell
In Categories versus Tags - What’s the Difference and Which One?, Lorelle makes the following points:

• "Categories help visitors find related information on your site. Tags help visitors find related information on your site and on other sites."
• "Categories generate a page of posts on your site. Tags can, too, but often generate a page of off-site posts on an off-site website".
• "Tagging gives you topical search capabilities for your site that are a middle ground between categories and all-out search, but it shouldn’t replace categories entirely."
• "Should tags replace categories? Absolutely not."
• "I use categories as broad groups of posts and tags as micro-groups of posts, helping narrow down the interest."
• "Tags shouldn’t replace categories, but they can help the user and search engines and directories find and catalog related information on your site."

Notes from Problogger
In Using Categories and Tags Effectively on Your Blog, Michael Martin makes the following points:

• "The number of categories should be small."
• "Each post goes into one category."
• "Use the same tags over and over again."
• "The tag cloud is easy to scan."

The End in Mind
In the ideal scenario, to use tags and categories more effectively (assuming your blogging platform supports it), you would have the following in place:

• A small set of categories for browsing the key themes of your site and for helping SEO (by having relevant category names in the full URL.)
• A nice tag cloud that helps users browser your site more like a topical search -- using words that your users would know and be looking for.
• Posts tagged with Technorati and del.icio.us tags that match the most relevant niches.

Turning It Into Action

• Use categories to divide your blog into a small set of mutually exclusive buckets.
• Use internal tags for slicing your content in more granular ways and to create tag clouds for your users.
• Tag your posts with external tags for Technorati and del.icio.us to reach the relevant social circles.

Additional Resources

• How Tags are Implemented in Wordpress 2.3
• Lorelle: Categories versus Tags - What’s the Difference and Which One?
• Organizing Information: Using Tags versus Categories
• Problogger: Using Categories and Tags Effectively on Your Blog
• Problogger: Tagging vs. Categories

My Related Posts

• Top 100 Blogs in Technorati