EDITOR ICONS
Default icons are stored in "icons" folder of the editor. It is configurable for each editor. If you want to use custom icons it is best to put them in files directory or your theme's directory.

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, or you may want to include some editor specific css files for theming purposes. For all these, you can use the library settings of the editor.