Linebreaks

End a line with two spaces to add a
linebreak:

How do I love thee?   
Let me count the ways

Italics and Bold

*This is italicized*, and so is _this_.
**This is bold**, and so is __this__.
Use ***italics and bold together*** if you ___have to___.

Links

There are three ways to write links. Each is easier to read than the last:

Here's an inline link to [Google](http://www.google.com/).
Here's a reference-style link to [Google][1].
Here's a very readable link to [Yahoo!][yahoo].

Bare URLs

We have modified our Markdown parser to support "naked" URLs (in most but not all cases -- beware of unusual characters in your URLs); they will be converted to links automatically:

I often visit http://example.com.

Force URLs by enclosing them in angle brackets:

Have you seen <http://example.com>?

URLs can be relative or full.

Headers

Underline text to make the two

top-level headers :

Header 1
========

Header 2
--------

Horizontal Rules

Insert a horizontal rule

---

by putting three or more hyphens, asterisks, or underscores on a line by themselves:

---

Simple lists

- Use a minus sign for a bullet
+ Or plus sign
* Or an asterisk

1. Numbered lists are easy
2. Markdown keeps track of the numbers for you
7. So this will be item 3.

Advanced lists: Nesting

1. Lists in a list item:
   • Indented four spaces.
     • indented eight spaces.
   • Four spaces again.

Simple blockquotes

Add a > to the beginning of any line to create a

.

> The syntax is based on the way email programs
> usually do quotations. You don't need to hard-wrap
> the paragraphs in your blockquotes, but it looks much nicer if you do.  Depends how lazy you feel.

Advanced blockquotes: Nesting

To put other Markdown blocks in a < blockquote >, just add a > followed by a space.

Images

Images are exactly like links, but they have an exclamation point in front of them:

![Valid XHTML](http://w3.org/Icons/valid-xhtml10).

Inline HTML

If you need to do something that Markdown can't handle, use HTML. Note that we only support a very strict subset of HTML!

To reboot your computer, press <kbd>ctrl</kbd>+<kbd>alt</kbd>+<kbd>del</kbd>.

From : http://stackoverflow.com/editing-help