The text files are files broken in two parts: the title and the body. The format is very simple - the title is always the first line of the text file and all other lines are the body. The body is html formatted text (no automatic breaks are added - the html is copied as-is to the output files) without the <html><body> stuff. The body also contains html comments inside <-- and --> which contain directives that alter the page's appearance (like the icon in the tree) or its position in the parent group.
An example
Directory structure
<!--@weight=200-->
<p>A content directory must have a specific directory (folder) layout, which mimics the contents of the help site's tree. The directory is made up of one or more text files and subdirectories. Each text file represents a simple page and contains html code and <a href='../../reference/directives.html'>directives</a> for the QuHelp system. A folder represents a group of pages (in the default template the folders are represented as books). The folder itself can have content. Since a directory in the operating system cannot have content by itself, a special text file called <tt>info.txt</tt> is used for the folder's content. This text file is not shown in the tree but its contents are used for the folder's contents.</p>
The line in red is the page's title. The line in green is a directive to set the page's weight to 200 (see Content files about page weights and ordering) and lastly the blue parts are html tags.