Markdown is a way of formatting text and inserting images. It can be used within Habitica in all of the Chat features found in the Social tab, and also in the titles and notes of Tasks, Rewards, and Checklists.
The images inserted with Markdown can be hosted on any web site. (Emoji can also be used to insert small images without using Markdown, but the selection of images is limited.)
Note that in the mobile apps, the formatting produced by Markdown (as shown in the table below) may not display as expected.
As shown in the table below, headers of different levels can be created by preceding the text with one or more # (hash marks). The smallest header uses six hash marks. The hash marks must be at the start of a line and there must be a space between them and the text of the header; otherwise, the hash marks will remain visible and the text will not be enlarged. In chat, use headers only when essential because they take up a lot of space.
Know more about Markdown see here http://habitica.wikia.com/wiki/Markdown_Cheat_Sheet