Markdown was created with the intention to style the text as you are creating it. It's simple because if you need to emphasize a word or sentence you only need to wrap it between two * or any other character of your choosing. If you need to read more about the syntax or the basic you can do it over at Daring Fireball.
Markdown is simple. Markdown lets you style the text without removing the hands from the keyboard for unnecessary features. Because you could start writing in markdown even in a simple text file, you can start focusing on what matters: the content. Because of its simplicity and increased quality of the resulting text it has gained a lot of users, mainly among the programmers.
There's probably at least one plugin for your favourite CMS (Wordpress, Drupal, etc.) and if writing is your second nature and you have trouble focusing, then you can give Write Monkey a try.