Add/delete blog tags
From MoodleDocs
A tag is a relevant keyword or term associated with a blog entry, describing it and enabling keyword-based classification of information for the purpose of retrieval. Typically, a blog entry will have one or more tags associated with it.
To encourage the use of blogs in your course, you may wish to add a Blog Tags block to your course page.
User Defined Tags are used for what? User Defined Tags can be used by anyone in the site. A user can only delete their own user-defined tag.
Official Tags are used for what? Official Tags are defined by the Site Administrator.