See Setup/R Markdown.
Wrap a word with single asterisks for italics and double asterisks for bold.
Wrap a word with single asterisks for *italics* and double asterisks for **bold**.
Surround inline code chunks with single
`backticks` or full code blocks with triple backticks:
``` yum <- is.delicious('gorgonzola') yum  TRUE ```
Lots of options are available for generating markdown tables from R objects, or text.
Note: differences between these packages are mainly in syntax but more importantly also in the possible output formats. Some only work with HTML output, some only PDF, etc.
If this is a manuscript, and you are noticing (especially in Git) everything is on a few really long lines, simply reflow using the shortcut (CTRL+SHIFT+/) or remember to hit enter at the end of a line.