Categorising posts, and the difference between categories and tags, and how they all relate to SEO, are all questions that we often get asked when doing iChurch training. Here’s a great article from the Elegant Themes blog which I find explains these things really clearly
http://www.elegantthemes.com/blog/tips-tricks/best-practices-for-using-categories-and-tags-in-wordpress
and another from Theme Grill:
https://themegrill.com/blog/using-wordpress-tags-and-categories-for-seo/
we hope this helps!