Markdown

Markdown is a specific markup library and text-to-HTML conversion tool for web writers. Markdown allows us to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).
Although Markdown is not a replacement of HTML, it is especially useful for creating simple articles like writing blogs, forums readme files etc.
Below is a syntax comparison

MarkdownHTMLcompiled output in browser
HeadingThis is a heading
==============
<h1>This is a heading</h1>

This is a heading

Heading (alternative)#This is a heading<h1>This is a heading</h1>

This is a heading

Sub HeadingThis is a sub heading
---------------
<h2>This is a sub heading</h2>

This is a sub heading

Sub Heading (alternative)##This is a sub heading<h2>This is a sub heading</h2>

This is a sub heading

H3 - H6###Match the number of # to the heading level<h3>Match the number of # to the heading level</h3>

Match the number of # to the heading level

ParagraphsParagraphs are separated

by a blank line.
<p>Paragraphs are separated</p><p>by a blank line.</p>

Paragraphs are separated

by a blank line.

Line Breaks Line breaks are represented by two spaces&npsp; 
at the end of a line.
<p>Line breaks are represented by two spaces<br>at the end of a line.</p>

Line breaks are represented by two spaces
at the end of a line.

Text attributes*italic*, **bold**, `monospace`, ~~strikethrough~~ <i>italic</i>,<b>bold</b>,<code>monospace</code>,<s>strikethrough</s>italic,bold,monospace,strikethrough
LinkA [link](http://example.com) A <a href="http://example.com">link</a>A link
Unordered List * Option 1
* Option 2
* Option 3
<ul><li>Option 1</li><li>Option 2</li><li>Option 3</li></ul>
  • Option 1
  • Option 2
  • Option 3
Ordered List 1. Option 1
2. Option 2
3. Option 3
<ol><li>Option 1</li><li>Option 2</li><li>Option 3</li></ol>
  1. Option 1
  2. Option 2
  3. Option 3