Markdown is a way of adding formatting to your mod listing, including headers and code and embedded videos, images, etc. You should definitely use them to spice up mod listings. The full specification is here, but here's a quick overview:
Quickest way to get started is an example.
## My super cool mod This is a very neat mod I wrote that lets you do *super cool things*. **Features** * Adds more parts * Adds more science * Adds more fun **Installation** 1. Copy the files into GameData 2. Say a little prayer 3. Make a little love 4. Get down tonight [Forum Thread](http://forum.kerbalspaceprogram.com/threads/84154)
Given that markdown, you'd get this content:
My super cool mod
This is a very neat mod I wrote that lets you do super cool things.
- Adds more parts
- Adds more science
- Adds more fun
- Copy the files into GameData
- Say a little prayer
- Make a little love
- Get down tonight
You can use Markdown on your mod descriptions, profile page, and changelogs. The nice thing about using it in your changelogs is that it looks nice in plaintext, which is how it shows up in your user's email inboxes.
You can easily embed a single image like so (this came from the Markdown spec):
You can also embed YouTube videos and Imgur albums by wrapping the URL in brackets (specific to SpaceDock):