WordPress is an Open Source software system used by millions of people around the world to create websites and blogs. Thousands of user guides & documentation for WordPress as well as blog posts, tutorials and reviews of themes & plugins.
It is completely customizable by the use of themes and plugins (both free & paid). Themes can be easily downloaded from the official WordPress site or from hundreds of other places around the web. The same goes with plugins, which are used to extend the functionality of your WordPress site