Note: Custom icon and library paths were introduced and temp directory was removed in 6.x.

EDITOR ICONS
Icons are stored in "icons" folder of the editor. You can put additional icons into this folder to make them visible
in the icon list in the editor edit page.

EDITOR LIBRARY
While creating a javascript button you may want to use functions or variables from an external javascript library,
in order to shorten the content text and make it clean. In this case, you should place your javascript library inside
the "library" folder of the editor. The .js files in this directory will be automatically loaded together with the editor.
Note that library/...js files are common and are loaded for every editor you define.
In case you want a library file to be loaded for only a specific editor, you should create a sub-folder under library
directory, which has the same name as the editor. For example, library/editorFoo/...js files will only be loaded
for the editor having the name "editorFoo".

EDITOR TEMP DIRECTORY
After starting to use the editor, you might notice that an extra folder named as bueditor in file system directory is created. This directory is used to store javascript files that contain buttons of your editors. Including them in a file instead of including them inline, allows browser caching and reduces page load time.
Since buttons may have php code that makes them change dynamically, md5 is used to track the changes in buttons script. If there is a matching file in the temp folder, it is loaded. Otherwise, a new file is created and loaded. If file loading fails, buttons are included as inline script.
Note: Files older than 15 days are cleaned in each cron run.
If download method is PRIVATE, buttons are always included inline.

Under supervision of Drupal :)