From the perspective of a regular PHP/MySQL programmer, categories and tags are almost identical. On the average blog, categories should come in a fixed number. The categories should cover all general aspects of your blog topic, and, as a result, their number should be small.

To help users easily find an article, you should post it into a single category. All blog platforms consider categories a navigation menu. The categories replaced the regular pages, even if this option still exists.

What if your article fits into multiple categories? Well, you either made a mistake by creating similar categories, or your article covers more than one topic (and you can split it into two articles), or you can use tags. The new tags functions in WordPress 2.3 are a perfect way of creating a subcategory navigation structure (not menu).

Use topic-related tags, do not use more than 20 tags on a small to medium blog and use a tag cloud, which, in fact, is a “Popular Searches” feature. The more popular a tag is, the bigger it appears in the cloud, and viceversa.

Example: you have an article about WordPress blogging. Your blog also features articles about other platforms, such as Blogger, Movable Type, and more. So, you have a large “Blogging” category, right? Well, articles about WordPress will be tagged with “blogging”, “wordpress”, while those about Blogger will be tagged with “blogging” and “blogger”.

Try using a concise category menu in your sidebar and a tag cloud under it. Your navigation behaviour will improve noticeably.