What is WordPress Gutenberg?

What is WordPress Gutenberg and what does it allow WordPress developers do?

Gutenberg (WP version 5.*)

Gutenberg is the name of a major WordPress update (version 5.0).

The Gutenberg update uses React.js as part of its framework, in order for developers to create custom blocks, which can be used within the content editor in WordPress.

With the introduction of Gutenberg, it helps WordPress compete with page builder CMSs, such as Wix or Squarespace for example.