Posts in Discourse at Mark’s Math are written in Markdown - a lightweight, easy to read/write syntax to indicate how text should be nicely formatted.
For example, if I wanted my rendered text to look like this:
Reasons Markdown is awesome
Markdown is really awesome because
- it’s easy to read,
- it’s easy to write,
but that’s all?
- Actually, there’s a lot more, like the output is awesome!
Reasons markdown is not awesome
- NONE!
I could type:
## Reasons Markdown is awesome
**Markdown** is really awesome because
- it's easy to *read*,
- it's easy to *write*,
- ~~but that's all?~~
- Actually, there's a lot more, like the output is **awesome**!
## Reasons markdown is not awesome
- **NONE!**
Note how it’s just basic text with a few extra symbols like
**bold**for bold*italic*for italic.~~strike through~~forstrike through
Higher level items include hashtags (like ##) for sections and dashes (like -) for list items. You can take a look at the Markdown cheatsheet already linked for a more complete introduction.
The toolbar
When you start a post by hitting either the “New Topic” button on the front page or by hitting a “Reply” button when reading a post, an editor panel should slide up from the bottom of the page. Near the top of that window, you should see a toolbar that looks something like so:
You can use that as a Markdown assistant. Clicking on the B at the left, for example, inserts the ** and ** that you need for bold text.
Images
The 
icon that I highlighted in the toolbar image above is particularly convenient for including images. When you click on it, you can choose an image from your machine, that image will be uploaded, and the proper Markdown syntax will be inserted into your post.
That’s how I inserted the image of the toolbar in the first place!
Dynamic images with Desmos
If you use Desmos, you can create really nice images and make them interactive as well. Here’s an example
Note that you can move the h-slider to illustrate one of the fundamental ideas of calculus. The image is a direct embedding of this Desmos graph.
To embed any Desmos graph, simply click the share 
button in the top right of the Desmos window.