This question really confused me. I am using since 2 years but not indexing and no follow to Google.
Lots of people searching about this I have found the good source about this I am going to share here.
If your website as a book then Categories should be Table of contents.
How to use WordPress tags and categories
and Tags are Index of your book.
DO NOT USE Tags as keywords.
If you are writing about foods, you ate on your personal blog. categories should be Food, Music, Early Life Interest etc.
The post about what your eats. and category Food, and tags like pizza, burger, Roasted Chicken etc
- what about duplicate content on category and tags archive page and post page.?
if you do not use this effectively there is a chance for duplicate content.
Finally, I understood half it. if you have the suggestion please comment here.