WordPress powers over 30% of all websites live on the Internet
WordPress is what is referred to as a CMS, or Content Management System, named such as it allows you to easily manage your content. It's not the only CMS out there, there are hundreds, but we feel for most people's needs, it has the power and flexibility to do whatever you need.
Years back, WordPress was the most powerful blogging software tool available. It has come a long way since then, and now you get both all in one platform. It's open source, meaning no one owns it and thousands of developers all over the world contribute to its features and improvements.
Since it is open source, it is not proprietary, allowing you to move your content or have any number of developers maintain and improve your site.
Don't confuse WordPress.com which is a site builder, with WordPress managed software, the name is about the only similarity.