A list of the most common formatting rules. There also is a complete list at TextFormattingRules.

(empty line)     Make a paragraph break. 
----             Horizontal ruler
[link]           Create hyperlink to "link", where "link" can be either an internal 
                 WikiName or an external link (http://)
[text|link]      Create a hyperlink where the link text is different from the actual 
                 hyperlink link.
[link#heading]   Jumps to that heading on the page.  The heading here needs to be 
                 written in CamelCase.
[text|wiki:link] Create a hyperlink where the link text is different from the 
                 actual hyperlink link, and the hyperlink points to a named Wiki. 
                 This supports interWiki linking.

*                Make a bulleted list (must be in first column). Use more (**)
                 for deeper indentations.
#                Make a numbered list (must be in first column). Use more (##, ###) 
                 for deeper indentations.

!, !!, !!!       Start a line with an exclamation mark (!) to make a heading. 
                 More exclamation marks mean bigger headings.

__text__         Makes text bold.
''text''         Makes text in italics (notice that these are single quotes ('))
{{text}}         Makes text in monospaced font.
;term:def        Defines 'term' with 'def'.  Use this with empty 'term' to 
                  make short comments.
\\               Forced line break (please use sparingly).

Don't use HTML, it doesn't work.

If you want to use images on your page, simply use a wikilink, like [myImage.png], save the page, and attache the image to the page (of cours with the exact same page). You can find the list of approved formats, on the SystemInfo page.

To make a code block, use triple {'s to open, and triple }'s to close.

If you want to use square brackets ([]) in the page without creating a hyperlink, use two opening square brackets. So the text [[Example Non-Link], will appear as [Example Non-Link].

(Wondering where this text comes from? It's on a page called Edit Page Help, which you can edit too!)