Don't people just use WordPress now?

WordPress is a content management system (CMS), intended for the development of blogs and small/simple websites (e.g. your local boutique). All websites are constructed using any number of various languages (HTML, CSS, JS, PHP, Node, Java, Ruby, etc) - WordPress is a popular choice for small-midsize projects because it allows your clients (the local boutique, for example) to maintain the website after you build it. You still need to know a fair amount of code (namely, HTML/CSS, PHP, & JS) - but your clients won't need to.

Markdown is a markup language that you can use to quickly and easily write simple HTML content.

For example, in the above paragraph, I italicized the words "markup language". I did so by surrounding the words with an asterisk (*). This is much more readable (and faster to type) than using HTML to italicize the words.

So there are a few things to address in your post:

1. WordPress and Markdown aren't really comparable as they aren't the same thing. The only similarity between the two is that they both save you time and generate code. FYI this reply to your answer has been done using Markdown :)
2. WordPress is built on top of several languages (detailed above) - you will need to learn those if you wish to be competent at working with WordPress. In learning them you may even realize why many devs use WP (and why the majority of us try to avoid it).
3. All of this information you can find at Team Treehouse, in the courses they offer! It sounds like you're looking to get started in Front-End Web Development, though you may wish to start with a deeper look at the basics of web coding with Web Design.

Good luck!