As the filler help text notes when you begin composing a post, we support Markdown and BBCode.
Unfortunately, there isn’t an official built in formatting reference yet.
The best Markdown reference I have found is at Stack Exchange:
The official BBCode reference is here:
(Note that we only support BBCode as a shortcut aid to learning Markdown, it’s not really “native” … so try to learn Markdown.)
A strict subset of some HTML also works, I can type:
this is <b>bold</b>
and get
this is bold.
But the HTML should really be avoided in favor of Markdown since it is more human readable when editing.