Markdown is the language and short codes that you'll need to use to write on Ghost. It's not too complex and is highly powerful once mastered.

If you'd like to know a bunch about it, here's a great post on Daring Fireball that goes into it in quite some detail.

Here's the list of the most popular markdown codes, it's result and the shortcuts (it's available inside each Ghost post by click on the ? in the top right hand corner of the markdown box):

Here's an awesome cheat sheet created by Theme Spectre:

They've also made it available as a PDF in case you want to print it and put it up somewhere, so I've attached it below.