WordPress is a free software that you install on your servers to create your website. It is known as a ‘blogging’ platform, but many people use it for their regular websites.
It has the ability to give you pages and posts. Pages are considered ‘static’ content, while posts show up in your RSS feed.
WordPress is a very Search Engine Friendly system, and is very interactive with the ability for your visitors to leave comments. Christina Hills teaches WordPress in the Website Creation Workshop.
Here is the Wikipedia definition of WordPress
Now if you want to know the difference between WordPress.org and WordPress.com watch the video below