WordPress became one of the most common and most versatile website platforms in the world. The fact that it is open source and free, made it the most widely used content management system. The number of design and functionality plugins is enourmous. It can be used to start a small blog or a heavy duty website. The options are limitless with WordPress.
Designs & Themes
What has made WordPress so successful was it's flexibility in design. By using a modular system the designs of a website could be exchange by using themes that were available for free in the WordPress database. As soon as paid premium themes were made available to the WordPress community, the popularity of the WordPress content management system exploded.
functinality & Plugins
The other component that significantly contributed to the success of the WordPress CMS was the possibility to expand its functionaliy buy using the so called plugins. These additional pieces of software were made available by thousands of developers around the world and could be download and installed directly inside the WordPress admin area. As soon as premium plugins came on the market, very sophisticated extension were made available, almost making web developers redundent.