WordPress is bundled with a customized version of the open source WYSIWYG editor TinyMCE by Moxiecode Systems, AB. Some questions about TinyMCE can be directed to Moxiecode's forums. However, since the WordPress 2+ version is custom Moxiecode won't be able to offer complete support. Here's a bit more information about TinyMCE and the WordPress version.
TinyMCE is a javascript application that offers a great deal of extensible functionality related to WYSIWYG text editing. Specifically TinyMCE allows for extension through plugins and themes, and allows the host of the TinyMCE install to control what functions are available to the users outside of a full-fledged theme by adding or removing buttons.
TinyMCE's configuration is determined when the WordPress edit page is loaded and are controlled by options set in the TinyMCE initialization process. More documentation on the configuration options can be found in the TinyMCE Configuration Reference.
In the WordPress version of TinyMCE, these options are set in a file entitled tiny_mce_gzip.php, which can be found in %WordPress_root%/wp-includes/js/tinymce.