WordPress is an open source blog tool and publishing platform powered by PHP and MySQL. It’s often customized into a Content Management System (CMS) where end users can manage their dynamic content themselves without having a knowledge of PHP or SQL. It has many features including a plug-in architecture and a theme template system.

WordPress is used by over 14% of the 1,000,000 biggest websites with the a whooping market share percentage which is 54% in overall CMS usage.

The best thing about WordPress is the it has a simple theme structure which anyone who has a knowledge of HTML and a bit PHP can create with so much of ease.

See the detailed anatomy of WordPress theme which explains every bit of wordpress theme structure in details:

via Yoast