WordPress is an open source CMS platform. Over 20% of the websites in the world are run on a WordPress platform.
Originally built as a blogging platform, WordPress now allows you to have increased functionality through the many plugins available.
The look and the feel of a WordPress website is in the theme, with many themes also including functionality not available in the straight WordPress application.
WordPress comes into its own for clients that wish to regularly blog while leaving the majority of the pages static.