What is Gutenberg? Gutenberg is the new default text editor for WordPress, set to arrive some time in 2018. It will be replacing the current TinyMCE editor.
On WordPress.com, its arrival will be dependent on when it is integrated into each of the currently offered themes. For VIP clients it’s integration will be managed allowing an easy opt-in process.
For sites using the software from WordPress.org, it is scheduled for inclusion in WordPress 5.0 (the exact date for this hasn’t been determined yet). You can test it out using the Gutenberg plugin (ideally just on test sites).
The aim of Gutenberg is to make the publishing experience simpler, by dividing everything up into blocks.
Ultimately the goal of Gutenberg is to move beyond simply using blocks in a page or post editor, to eventually building your whole site with it.
It is also a very exciting time for developers who want to get their hands dirty. Existing themes can be made compatible with Gutenberg, new themes have a much more standard way to create rich post layouts and the instructions for users on how to set things up will be much clearer in the interface. Plugin developers can use blocks in a multitude of ways to extend WordPress.
If you want to learn more about how Gutenberg works, check out the Gutenberg page over at WordPress.org. CodeinWP have created an extensive write-up, and the folk over at Gutenberg.news are keeping a great up-to-date blog with recent Gutenberg news.