Tags are simply 'keywords' for that particular post. They are
similar to labels or categories, but are more specific. You can
imagine them (categories, labels and tags as filters).
Categories or Labels in a blog cover a broad criteria or
subject.
Tags is a blog cover a specific criteria or keyword.
Fog e.g., a blog may contain 500 posts on 'Sports' - say 100
each on Golf, Motor Racing, Baseball, Basketball and the rest on
others. So the word 'sports' is a category or label.
Within the Baseball posts, there might be 11 posts on 'Yankees'
and 10 on 'Mets'. When you tag a post with those 'Tags' or
keywords, a reader will be able to quickly see all the posts tagged
with Yankees or Mets.
Taking it even further, tagging Yankees posts about Alex
Rodriguez with his name would enable a reader to specifically read
posts about Rodriguez.