The Story Behind the New WordPress.com 11 years ago
It's a web app too. Wordpress.com has been steadily introducing features (through Jetpack) that allow you to manage your installed Wordpress instances - this brings the two together.
So you can login to wordpress.com either via the web, apps, Air etc to manage/publish to any types of Wordpress installs.
It means that you don't have to use the old wp-admin CMS and can use what they've now built and released.