WordPress History
WordPress was created in 2003 by Mike Little and Matt Mullenweg. WordPress was originally created as a website blogging platform, but over time it developed to cover more needs and is a very popular CMS today. 43% of websites are powered by WordPress today and it is free for anyone to download.
WordPress Entity
WordPress.org is WordPress's main website and is a non-profit organization. You can download all the files for WordPress there and it has tons of resources for developers on the website. When you download WordPress from WordPress.org, it does not provide hosting and that is something the user would need to find on their own.
WordPress.com Entity
Anyone can also download files from WordPress.com, but at a price. WordPress.com takes care of the hosting for a user, so that you don't have to worry about that part. Their prices range from $9-$70 per month depending on the type of plan and your needs. WordPress.com is is maintained by Automattic - a completely separate company that makes sure that WordPress.com can be the best that it can be.
Summary
Overall, I think WordPress is a great CMS option and you can't go wrong with either route - downloading it from WordPress.org and finding a host or purchasing it from WordPress.com. The nice thing about WordPress.com is that you don't have to worry about the hosting then and they take care of that.