Quoting the introduction on RStudio official site:

R Markdown is an authoring format that enables easy creation of dynamic documents, presentations, and reports from R. It combines the core syntax of markdown (an easy-to-write plain text format) with embedded R code chunks that are run so their output can be included in the final document. R Markdown documents are fully reproducible (they can be automatically regenerated whenever underlying R code or data changes).

When you use RMarkdown in R, you need to install this package manually. If you use RStudio, RMarkdown and knitr are pre-installed.
There is a knit button on top of the editor window which helps you to convert .Rmd file to specified format.

## Frequent Used Syntax

Per below you could find some frequent used syntax:

• # for header - level 1, one more # leads to one level down
• * or number like 1, 2, etc. at the beginning of a sentence for creating a list, unordered or ordered.
• *<blabla>* for italic, **<blabla>** for bold, _<blabla>_ for underscores, ~~<blabla>~~ for strikethrough.
• italic
• bold
• underscore
• strikethrough
• A pair of <your code>  for quoting a smaill piece of inline code, if you embed a piece of code in r  , R Markdown will run the code in between automatically and replace it with the result.
• A string of 3 backticks following {r} to start a chunk of code indicating the chunk of code is programmed by R. To end this chunk, you just need to add another 3 backticks.

• To create a block quotation, each line preceded by a > character and a space. (The > need not start at the left margin, but it should not be indented more than three spaces.)

• [website_name](http://...) set a hyperlink
• Use $$<equition>$$ to display equations in a new line and $<equition>$ for inline equation. You can use all of the standard latex math
symbols
to create attractive equations.
• Three or more asterisks ***** or dashes ----- are used as horizontal rule or page break.
• To create a table:

First Header  | Second Header
--------------|--------------
Content Cell  | Content Cell
Content Cell  | Content Cell


You could check the link for more basic syntax.

## Practical Syntax

### Chunk Options

A chunk option is inserted right beside {r <option>}.
For instance, {r message = FALSE} means all messages generated in the chunk is suppressed. Along with message, there are also warning and error which control the info delivery from the chunk. Besides, there are 3 options controlling code processing.

option code display code processing result display
echo=FALSE X O O
eval=FALSE O X X
results='hide' O O X

### Reference Label

An interesting feature available in knitr is the labeling of code snippets. The code chunk below would be assigned the label summary_mtcars_disp:

However, because the results option is equal to hide, no output is shown. This is what appears in the output document:

summary(mtcars\$disp)


What purpose do these labels serve? knitr provides the option ref.label to refer to previously defined and labeled code chunks. If used correctly, knitr will copy the code of the chunk you referred to and repeat it in the current code chunk. This feature enables you to separate R code and R output in the output document, without code duplication. Let’s continue the example; the following code chunk:

produces the output you would expect:

##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
##    71.1   120.8   196.3   230.7   326.0   472.0


Notice that the echo option was explicitly set to FALSE, suppressing the R code that generated the output. There is a myriad of options available for code chunks, you can discover them at Yihui Xie’s website

### Alternative Output Formats

#### Basic Options

When you create a .Rmd file, RStudio automatically generates a YAML field like:

---
title: "This is a title"
author: "I am the author"
date: "October 1, 2015"
output: html_document
---


The first 3 lines generates the title author and date in the output html document. There are 3 default output format: html_document, pdf_document and word_document. Besides, you can also set it to be md_document. The latter output method is very helpful when you want to display .Rmd in a static blog. (WordPress, Hexo, etc.)

Notice that to visualize data in a pdf document, you will have to use the ggplot2 package as an alternative to the ggvis package. This is for a reason: the ggvis package creates graphs that are HTML objects. These graphs are useful for HTML documents, but cannot be included in a pdf document without intermediary steps.

You can also export your file as a slideshow by changing the output field to one of beamer_presentation, ioslides_presentation and slidy_presentation.

which creates a slidy HTML slideshow. R Markdown will start a new slide at each first or second level header in your document. You can insert additional slide breaks with markdown’s horizontal rule syntax:

***


#### Specify knitr And pandoc Options

Each R Markdown output template is a collection of knitr and pandoc options. You can customize your output by overwriting the default options that come with the template. For example, the YAML header below overwrites the default code highlight style of the pdf_document template to create a document that uses the zenburn style:

---
title: "Demo"
output:
pdf_document:
highlight: zenburn
---


The YAML header below overwrites the default bootstrap CSS theme of the html_document template.

---
title: "Demo"
output:
html_document:
theme: spacelab
---


Pay close attention to the indentation of the options inside the YAML header; if you do not do this correctly, pandoc will not correctly understand your specifications. As an example, notice the difference between only specifying the output document to be HTML:

---
output: html_document
---


and specifying an HTML output document with a different theme:

---
output:
html_document:
theme: spacelab
---


Guide
.

#### Brand the Reports with Style Sheets

In the last exercise, we showed a way to change the CSS style of your HTML output: you can set the theme option of html_document to one of default, cerulean, journal, flatly, readable, spacelab, united, or cosmo. (Try it out). But what if you want to customize your CSS in more specific ways? You can do this by writing a .css file for your report and saving it in the same directory as the .Rmd file. To have your report use the CSS, set the css option of html_document to the file name, like this

---
title: "Demo"
output:
html_document:
css: styles.css
---


Custom CSS is an easy way to add branding to your reports.

#### Interactive Reports

shiny is an R package that uses R to build interactive web apps such as data explorers and dashboards. You can add shiny components to an R Markdown file to make an interactive document. You can also use R Markdown to create reports that use interactive ggvis graphics. ggvis relies on the shiny framework to create interactivity.

When you do this, you must ensure that you add runtime: shiny to the file’s YAML header. You need to use an HTML output format (like html_document, ioslides_presentation, or slidy_presentation).

To learn more about interactivity with Shiny, ggvis and R, visit Shiny in RStudio.

## Reference:

Reporting with R Markdown