WordPress is a free and open source dynamic content management system (CMS) and a blogging tool based on PHP and MySQL. It has many features including a plug-in architecture and a template system. WordPress is currently the most popular CMS in use on the Internet.
I work with the self hosted version available at wordpress.org. It is installed at your hosting site and allows the end user to manage their website on line with out having to know any code.
A theme is the look of your site and is install into the WordPress application. Once WordPress is installed and configured you will have a fully functioning website.