Many people are confused by the distinction between categories and tags in their WordPress blog. They sort of do the same thing, right?
Think of categories as the table of contents of your blog, while tags are the index. The category is the broad type of post, and the tags are the subject of that post. You should have just a few categories, and lots of tags.
If people subscribe to your mailing list, they might want to subscribe to just certain categories (as you can in the sidebar of this blog). You might like to read my website articles and not hear about anything else. But it makes little sense to subscribe to a tag* — like, you want to hear about everything that involves “pizza” whether it’s serious or silly?
If you already have a WordPress blog and have been using categories as if they were tags (as I did at first!), I recommend the plugin Categories to Tags Converter by wordpress.org as an easy way to fix things without editing all your posts manually.
* You can subscribe to a tag using an RSS reader, because every tag has a feed. It’s just, why would anyone want to?