WordPres.org:
This is the open-source version of WordPress, which you need to install on a server on your own. You will not have to pay for WordPress. When you take a look at “what is Wordpress org used for”, you will notice that most people who develop websites on their own with WP prefer to use it.
WordPress.com:
This is a free plan too that you have to buy from WordPress. It is easier to use, but you don’t have much flexibility as self-hosted WordPress.