It's a very fast HTML editor that runs in a browser and doesn't require any kind of installation on the client computer. CKEditor is a core module as of Drupal 8
Some features of the CKEditor module:
- ready to use plugins and buttons for <!--break-->and <!--pagebreak--> tags
- support for external file browsers, like CKFinder, IMCE
- linktocontent and linktonode plugins and buttons for easy linking to nodes or menu items.
- each user may have it's own configuration (choose skin, toolbar, language, default state of CKEditor, etc.)
- and much more!
CKEditor is compatible with most of the internet browsers, including
- Internet Explorer 6.0+
- Firefox 2.0+
- Safari 3.0+
- Opera 9.5+
- Google Chrome
- Camino 1.0+