What is a Tag?
The term tag has multiple meanings in computing.
A tag may be a text label that categories a post in a blog, news portal, social media, online community forum, or other website, e.g. posts that are reviews of movies may be tagged with the tag "Movie Reviews." From this sense comes the term hashtag (e.g. #moviereviews). Some Content Management Systems like WordPress support both "tags" and "categories" as separate concepts, and a single article could be tagged with both some tags and some categories. The different between tag and category depends entirely on the CMS. There could be no difference at all, or they could be used for completely different purposes.
A tag may be a tag of a file in your PC. Many filesystems support adding tags to files, however, because there many issues with this, so this sort of tag is seldom used, e.g. copying a file from an EXT4 filesystem to NTFS would lose the tag metadata.
A tag maybe also be a text code used to mark a piece of text is markup languages like HTML and XML, e.g. in <h1>My Review of The Metrix</h1>, <h1> is an opening tag and </h1> is a closing tag, together they mark the text between them as being a heading of a document. The same sense applies to other markup languages like BBCode, e.g. [b]bold text[/b].