Editors-xtd is the plugin group behind the buttons that appear with the article editor. They are extensions to the editor rather than part of it, so they only run when an editor plugin is active.
The buttons insert markers and links into the article for you rather than requiring the code to be typed. Between them they cover inserting an image from the Media manager, adding a Read more break, adding a page break to split a long article across several pages, and inserting a link to another article, a menu item or a module.
Each button is a separate plugin, so any of them can be switched off for a site that does not need it. They are listed together under System → Plugins when the list is filtered by the editors-xtd type.