The first thing you need to know is that WordPress is an open-source content management system (CMS), which is based on WordPress.org software.
To create a website using WordPress.org, you need to download the software, choose a website hosting provider (and install it on a web server). This is the reason why WordPress.org is commonly mentioned as “the self-hosted WordPress”.
WordPress.com is based on the WordPress.org but doesn’t require installing the software and self-hosting. You can sign up for WordPress.com right this second and have your website working immediately.