WordPress.com vs WordPress.org

Both the sites are part of WordPress. Only difference is about "who is hosting your website".

WordPress.com

WordPress.com is the site where you don't have to do anything. All the developing work will be handled itself. You don't have to purchase software, manage web server or pay for hosting.

There are some limitations with this site. For example, by default your domain name includes wordpress.com, you can't modify your site's coding, upload any themes or plugins. Your site will be free for the lifetime, but you have to pay to upgrade it for more specifications.

Upgrades also allow you to use a custom domain like you can use anything as the domain. Further upgradation also enhances appearance of your site, upload videos or photos and many more.

Your site will be secured and spam free as it will be under WordPress security. You only have to sign in and choose your blog name and focus on your content.

This site is preferred by everyone but mostly bloggers, photographers, artists, etc.

WordPress.org

WordPress.org is the site where you have to manage your own blog or website. You can find free WordPress software and install on your web server.

It gives full control and freedom over your site including the WordPress software. Most of the WordPress showcase sites are made on self hosting.

With full control it also places full responsibility of the site upon the user. From technical dealing to security issues each and everything has to be managed by the user itself.

WordPress.com vs WordPress.org