Markdown

Markdown is a superset of HTML, so any HTML file is valid Markdown:

<!--This means we can use HTML elements in Markdown, such as the comment
element, and they won't be affected by a markdown parser. However, if you
create an HTML element in your markdown file, you cannot use markdown syntax
within that element's contents.-->

You can create HTML elements <h1> through <h6> easily by prepending the text you want to be in that element by a number of hashes (#):

# This is an <h1>
## This is an <h2>
### This is an <h3>
#### This is an <h4>
##### This is an <h5>
###### This is an <h6>

Markdown also provides us with two alternative ways of indicating h1 and h2:

This is an h1
=============

This is an h2
-------------

Text can be easily styled as italic or bold using markdown:

*This text is in italics.*
_And so is this text._

**This text is in bold.**
__And so is this text.__

***This text is in both.***
**_As is this!_**
*__And this!__*

In GitHub Flavored Markdown, which is used to render markdown files on GitHub, we also have strikethrough:

~~This text is rendered with strikethrough.~~

Paragraphs are one or multiple adjacent lines of text separated by one or multiple blank lines:

This is a paragraph. I'm typing in a paragraph isn't this fun?

Now I'm in paragraph 2.
I'm still in paragraph 2 too!


I'm in paragraph three!

Should you ever want to insert an HTML <br /> tag, you can end a paragraph with two or more spaces and then begin a new paragraph:

I end with two spaces (highlight me to see them).

There's a <br /> above me!

Block quotes are easy and done with the > character:

> This is a block quote. You can either
> manually wrap your lines and put a `>` before every line or you can let your lines get really long and wrap on their own.
> It doesn't make a difference so long as they start with a `>`.

> You can also use more than one level
>> of indentation?
> How neat is that?

Unordered lists can be made using asterisks, pluses, or hyphens:

* Item
* Item
* Another item

or

+ Item
+ Item
+ One more item

or

- Item
- Item
- One last item

Ordered lists are done with a number followed by a period:

1. Item one
2. Item two
3. Item three

You don't even have to label the items correctly and Markdown will still render the numbers in order, but this may not be a good idea:

1. Item one
1. Item two
1. Item three

(this renders the same as the above example).

You can also use sublists:

1. Item one
2. Item two
3. Item three
    * Sub-item
    * Sub-item
4. Item four

There are even task lists. This creates HTML checkboxes:

Boxes below without the 'x' are unchecked HTML checkboxes.
- [ ] First task to complete.
- [ ] Second task that needs done
This checkbox below will be a checked HTML checkbox.
- [x] This task has been completed

You can indicate a code block (which uses the <code> element) by indenting a line with four spaces or a tab.

This is code
So is this

You can also re-tab (or add an additional four spaces) for indentation inside your code:

my_array.each do |item|
  puts item
end

Inline code can be created using the backtick character `:

John didn't even know what the `go_to()` function did!

In GitHub Flavored Markdown, you can use a special syntax for code:

```ruby
def foobar
  puts "Hello world!"
end
```

The above text doesn't require indenting, plus GitHub will use syntax highlighting of the language you specify after the opening ```.

Horizontal rules (<hr/>) are easily added with three or more asterisks or hyphens, with or without spaces:

***
---
- - -
****************

One of the best things about markdown is how easy it is to make links. Put the text to display in hard brackets [] followed by the url in parenthesis ():

[Click me!](http://test.com/)

You can also add a link title using quotes inside the parenthesis:

[Click me!](http://test.com/ "Link to Test.com")

Relative paths work too:

[Go to music](/music/).

Markdown also supports reference style links:

[Click this link][link1] for more info about it!
[Also check out this link][foobar] if you want to.

[link1]: http://test.com/ "Cool!"
[foobar]: http://foobar.biz/ "Alright!"

The title can also be in single quotes or in parenthesis, or omitted entirely. The references can be anywhere in your document and the reference IDs can be anything so long as they are unique.

There is also "implicit naming" which lets you use the link text as the id:

[This][] is a link.

[This]: http://thisisalink.com/

But it's not that commonly used.

Some Markdown flavors even make use of the combination of lists, links, and headings in order to create tables of contents. In this case, heading titles in lowercase are prepended with hash (#) and are used as link ids. Should the heading have multiple words, they will be connected with a hyphen (-), that also replaces some special characters. (Some other special characters are omitted though):

- [Heading](#heading)
- [Another heading](#another-heading)
- [Chapter](#chapter)
  - [Subchapter <h3 />](#subchapter-h3-)

Nonetheless, this is a feature that might not be working in all Markdown implementations the same way.

Images are done the same way as links, but with an exclamation point in front!

![This is the alt-attribute for my image](http://imgur.com/myimage.jpg "An optional title")

Reference style works as expected:

![This is the alt-attribute.][myimage]

[myimage]: relative/urls/cool/image.jpg "if you need a title, it's here"

Here are some other more niche features of Markdown:

In order to simplify work with Markdown and to unify its coding style, Markdownlint has been created. Available as a separate tool as well as a plugin for some IDEs, it can be used to ensure validity and readability of Markdown.

Most of these notes were pulled from learnxinyminutes so please go read more of their content.