title: Relationship between R Markdown, Knitr, Pandoc, and Bookdown
body: <p>What is the relationship between the functionality of R Markdown, Knitr, Pandoc, and Bookdown?</p>
  
  <p>Specifically what is the 'division of labour' between these packages in converting markup documents with embedded R code (e.g. <code>.Rnw</code> or <code>.Rmd</code>) into final outputs (e.g. <code>.pdf</code> or <code>.html</code>)?  And if Knitr is used to process RMarkdown, what does the <code>rmarkdown</code> package do and how is it different to the <code>markdown package</code>?</p>