By default, Drupal previously didn't come with any "What you See is What you Get" editor. Since Drupal 8 the core now includes the CK Editor. It's an open source system that doesn't have anything specific to do with Drupal but takes what you write with a list of plugin "buttons" on a toolbar and converts it into HTML that will be saved to your content and displayed on the web page once published.
CKEditor is a great tool to add word processing expectations to the CMS system, but it's not without its flaws or missing functionality. Since it's not specifically written for Drupal, Drupal has bridged some of the operations or image management, button controls or permissions, and file management into the Drupal system for upload, management and storage.
Issues or potential lacking features:
- Anchor links, target, and specific link attributes are lacking really any great functionality
- Image layouts and styles are incredibly basic
- Pasting from Microsoft word or other systems can have invisible formatting that it allows through the editor
- Since it's a small area to manage content, it can take a few times sometimes to edit → save content to see if laid out correctly.