HTML Editor
The HTML editor is an option for more advanced users which allows you to edit the HTML source of a field. It can be used to add special HTML markup to your cards that is not available via the GUI - for example a <hr> to add a horizontal line to a field.
The HTML editor can also be used to quickly edit formatting code, such as colours.
Note that due to technical limitations in the GUI's HTML renderer, complex HTML like tables will probably not turn out like you expect. See http://code.google.com/p/anki/issues/detail?id=410