I think of the difference between categories & tags this way:
Categories are used to define the general topics on your blog and tags are used to drill down a bit further into your content.
Categories create an outline of your site, providing a basic structure for your content. Tags are more specific, too specific to be a category themselves.
When to use categories & tags: an example
Let’s say I have a website about fashion. I might organize my posts into a handful of categories into which most of my posts would fit nicely. They might include:
- Wardrobe Must-Haves
So, let’s say I write a post about a great dress I found at Bloomingdales last weekend. I might write a post about it and add it to the category Shopping. I might tag it Bloomingdales.
This way, a reader could click on my Shopping category and find all my posts about my shopping excursions. If, however, they wanted to know more specifics about what kinds of things I buy at Bloomingdales, they might click on the Bloomingdales tag. Making a special category for Bloomingales would be impractical as I would then have to make separate categories for all the other stores I shop at. Major overkill.
How to Use Categories and Tags
I’m a fan of categories. In fact, I don’t think anyone would say you shouldn’t use categories on a typical blog for organizational purposes. Well-chosen and well-used categories can make a reader’s experience on your blog a pleasant experience.
I do not recommend using more than 5-7 categories.
Regarding tags, I think you should use as many as is necessary to keep your information organized without getting messy.