Mark Jaquith: Why WordPress Themes are derivates of WordPress
Back in 2010 Mark Jaquith posted an excellent article on "Why WordPress Themes are a derivate of WordPress" in his blog "Mark on WordPress".
Facts & Figures: 23 Percent of all Websites worldwide use WordPress
The latest usage data from W3Techs shows that 23,2 Percent of all Websites worldwide are using WordPress (as of November 8th, 2014).