Making content easy to edit with in a content management system usually involves the inclusion of a drop in JavaScript based Rich Text Editor. There's plenty of them around such as TinyMCE, FCKEditor, YUI Editor, to name but a few. Drupal has support for nearly all editors which are made available, but the level of integration varies greatly amongst the contributed modules.
With the arrival of Drupal 6 the WYSIWYG API module was born, with the task of unifying the configuration and integration of 3rd party JavaScript editors, and making it easy to get them installed on the server.