Docs: describe difference between tags and categories (#6882)

Merge pull request 6882

docs/_docs/posts.md

@@ -201,8 +201,32 @@ category: blog
 
 In this case, the listing pages will be accessible at `{baseurl}/category/blog.html`
 
-While this example is done with categories, you can easily extend your lists to
+Although categories and tags are very similar, they are used to group posts,
-filter by tags or any other variable created with extensions.
+there are a few differences between them.  Categories and sub-categories create
+hierarchies that, by default, are reflected in the directory structure of your
+site.  A post with the following header
+```yaml
+---
+layout: post
+title: A Trip
+category: [ blog, travel ]
+---
+```
+will be accessible at `{baseurl}/blog/travel/year/month/day/A-Trip.html`.  On
+the other hand, a tagged post
+```yaml
+---
+layout: post
+title: A Trip
+tags: [ blog, travel ]
+---
+```
+will be saved as `{baseurl}/year/month/day/A-Trip.html`.  It is up to you to
+create `{baseurl}/tag/blog.html` and `{baseurl}/tag/blog.html` the same way as
+described above for categories.
+
+While this example is done with tags and categories, you can easily extend your
+lists to filter by any other variable created with extensions.
 
 ## Post excerpts