Every post (and every page) has an editor. It’s the place where you can write content to be displayed for said post, change its title, permalink, terms associated with it, etc. The classic editor, as described below, was introduced into WordPress in 2005. It was a big improvement as it provided some WYSIWYG capacity that WordPress didn’t offer up to that point. This version of the editor was loved by many until it was pushed out by the block editor in 2018.
The classic editor is based on an open-source JavaScript library called TinyMCE. It provided an experience comparable to creating a document in Word. It had basically just one <textarea> with different controls, letting you add a bullet list, make the text bold, set a line to be a heading level 2, and much more. The visible content was then translated into HTML, which you could see if you switched your active tab from “Visual” to “Text” (or “Code” in modern versions of the editor). Take a look at what the classic editor looked like below.
Unfortunately, this editor was very limiting. The entire content of this text area was stored as a single blob of HTML in the post_content column in the database. If you wanted to add anything more complex than simple text and media, you had to use shortcodes (e.g., [my_gallery id=”123″]). These were placeholders, usually provided by plugins, that would be parsed and replaced with some HTML on the frontend.
A simple text editing experience like that was fine in 2005 when most websites were just text anyway, but as the web evolved, so did the requirements of the editor. This manifested itself with a booming rise of page builders, like Elementor or Divi, which often replaced the native editor altogether. WordPress tried to respond to that demand in 2018 by replacing the classic editor with the block editor. Today, you can only use the classic editor if you download a plugin for it.