WordPress is open source tool and its free and easy to use and a content management system (CMS). It based on PHP and MySQL.It runs on a web hosting service. Features include plug-in and a template system. WordPress is used more than 18.9% of top 10 million websites. WordPress is the most popular blogging system in Web and used in more than 60 million websites.
In WordPress users install themes.There are three default themes which are in WordPress when you install WordPress. Themes allow users to change the look and functionality of a WordPress .Install the themes and their content. Themes are install using the WordPress “Appearance” administration tool or uploaded on theme folder via FTP. Thousands of WordPress themes are there and some are free and some are premium (paid for) templates.
The downloading link of WordPress is : http://wordpress.org/download/