Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).
Writing blog articles with Markdown is more natural and streamline. Writing blog articles with Markdown is faster than with a WYSIWYG editor. Most importantly, Markdown increases significantly the joy of writing blog articles. Additionally, it’s also more easier to write blog articles offline, e.g. in a train.
Example for lists, strong and emphasis:
* Alice is a **woman** * Bob is a _man_ * Chris is a _**child**_
- Alice is a woman
- Bob is a man
- Chris is a child
Markdown is supported on all major CMS and blog platforms such as Drupal.
There are other markup languages such as Textile (Drupal Textile module). Textile resembles more the HTML tag structure, whereas Markdown is easier to read standalone. If you want a shorthand for HTML, go with Textile. See the Markdown vs Textile comparison.
There are different “dialects” of Markdown such as Markdown Extra for PHP.
I recommend Markdown to every blogger. Have a look at it.