Blogdown is both an addition to markdown and a tool for generating html from said markdown. As you can probably guess, blogdown is designed for blog posts. "But wait!" I hear you scream. Isn't the original markdown for blogging? Yes. Blogdown just adds some metadata to each blog post. Basically, it's a series of tags that get added to the top of a markdown post.
title: This is a title author: Brian tags: blogging, metablogging status: draft This is my blog post. I hope you like it.
There's a number of tags that are valid in "standard blogdown", they are: title, status, author, tags, published, updated, slug, and summary. Each of them add information about the post that can be used when generating the html for a given post.