Wysiwyg API allows to use client-side editors (a.k.a.  WYSIWYG editors) for
editing content in the Drupal CMS.  It simplifies installation of editors
and allows you to define which editor to use depending on the input format.
This module replaces all existing editor integration modules and no other
Drupal module is required.

It is capable of supporting any kind of client-side editor as long as there
are support files for it that integrate the external library with Wysiwyg
API.  A client-side editor can be a regular HTML-based editor, a
"pseudo-editor" (that just provides buttons to insert HTML markup into a
plain textarea), or even a Flash-based editor.  Support for various editor
libraries is built-in.

The Wysiwyg API also allows Drupal modules to register plugins (or
"buttons") for editors.