If you use a key word “category vs tag” to conduct a Google seach, most likely, the first article you found is a Wordpress article Categories vs. Tags.

The explanation in that article is also applied to my blog site. To be clear, I plan to use them in the following ways:

• Category is the first level and general categorization.
• Tag will be used as a more detailed categorization. Since there is no sub-category concept in my blog site, I may use tags as sub-categories.
• Both categories and tags are optional in my blogs. For better categorization, any my post will use one or both of them.