1. WordPress is not (only) WP-Admin
At its core, WordPress is a content management system, or CMS. Like any CMS, there is an admin side for creating and editing content, and a display side that renders the website. These two sides can be connected in different ways. For WordPress, the connection has traditionally been that the content editing side (also known as “WP-Admin”) stores content in a MySQL database, and the website/blog side makes pages by filling in templates with that content.
2. WP-Admin is tightly linked to WordPress
The direct connection used by WP-Admin requires special access to the database. That means that WP-Admin needs to be installed right along side the WordPress site it manages. It also limits WP-Admin to managing that single WordPress site.
3. Calypso is a new admin tool
Calypso is a fundamental rethinking of what the admin side of a WordPress site can be. Rather than connecting directly to the MySQL database, Calypso manages content over the web using a special language known as a REST API. This difference allows Calypso to be installed anywhere and to connect to any WordPress site, or even multiple sites. (With the right username and password, of course.)
4. Calypso is also for reading
A core part of the WordPress.com experience is following other authors. That means that Calypso comes with a “reader” baked in. From the prospective of a WordPress site owner, this is a big departure from WP-Admin where the focus was on your content.
5. PHP-based WordPress isn’t going anywhere
Calypso doesn’t really do much without a WordPress site to talk to. If you open the app (or download the source code and run it), you’ll be greeted by a very nicely-designed webpage asking you to sign in to a WordPress site. That’s because Calypso isn’t a CMS. It’s an admin tool that can talk to any CMS that speaks the right language (that is, has a compatible REST API). And right now, the only CMS that speaks that language is good old (PHP-based) WordPress.
6. Neither is WP-Admin
In its current form, Calypso is an alternative to WP-Admin, not a replacement. Whether it eventually replaces WP-Admin is up to the community of contributors to the WordPress (.org) project. The big sticking point is likely to be third-party plugins. Although you can enable/disable and update plugins using Calypso, many plugins extend WP-Admin to add their own settings pages.
Also published on Medium.