WordPress.org, often called self-hosted WordPress, is the free, open-source WordPress software that you can install on your own web host to create a website that’s 100% your own.
WordPress.com is a for-profit, paid service that is powered by the WordPress.org software. It’s simple to use, but you lose much of the flexibility of the self-hosted WordPress.
Most of the time, when people say “WordPress”, they mean the self-hosted WordPress available at WordPress.org. If you want to truly own your website, self-hosted WordPress.org is almost always the best option.