In my experience, this is one of the most under used features of a website is the ability to specify Categories and Tags.
First, the difference.
Tags are used to highlight key things in your content. Whereas Categories are used to describe your content. But both are used by Google and, when used in moderation and have a direct and measurable impact on what you are found for when someone searches for you.
Now, the benefit.
If you specify Categories or Tags, I've never seen someone not appear for those exact terms (in over ten years of doing this). This, of course, assumes you have a well optimized site with traffic that is similar to the top 3-5 other sites that appear for those terms on the first page of Google.
Then how to use them.
I made oatmeal this morning. I put a little too much hot water in it so it was diluted and the flavor wasn't all that good. But most days I get it just right. Well, Google see's your Categories and Tags the same way. The fewer terms you use the more likely you are to be taken seriously by Google. If you only had 5 Tags and 1 Category Google is going to say "They feel they're most relevant here. So let's run them through our algorithm and if they are then let's focus their points towards ranking in those terms." Otherwise, Google see's a huge list and can't decipher, as easily, what you're most relevant too, thus you rank lower.
And now where to use them.
Use Categories anywhere your Posts appear. If that's the front page of your website in a Recent Posts section or on the Single Post Page itself.
Ex.
Post Title by Author
My first category, my second category.
My excerpt or content.
Use Tags only on the Single Post page. You can use it in the same places as your Categories, but it's risky. If you do, you should A / B test so you can track the results you get with and without them.
One last tip.
Okay two last tips.
Use HTML5 microdata (contact us if you'd like more advice about this).
Ensure that any time the Categories or Tags appear that they're linked to a page that lists all other Posts in that Category or Tag (see the bottom of this article for an example).
Look up your competitors and see what they're Categorizing and Tagging their content with.
