What is exactly richtext and how do I format data in richtext in QTextEdit?
QTextEdit
can display text that is formatted with additional layout and style information (i.e. enriched) which can be set either programatically or by using a subset of HTML.
The Rich Text Processing documentation gives you an overview and pointers to more in-depth information.
If you're looking for a WYSIWYG toolbar for the QTextEdit, you'll end up disappointed. There is no default one (although the SDK contains an example app). So you should either make the toolbar yourself or find one.
The internal rich text format is tag/attribute-based, and is similar to HTML using in-line CSS style="xxx" attributes. The default export/import format (using toHTML/setHTML) is a subset of HTML. See this link:
http://doc.trolltech.com/4.6/richtext-html-subset.html
Note that CSS classes are not supported internally and are converted to their representing attributes at import.