WordPress is a free and open source blogging tool and content management system (CMS) powered by PHP (programming language for web development) and MySQL(database used for web applications).
What is a Theme? Themes allow users to change the look and functionality of a WordPress website without changing the informational content.
What is a Plugin? Plugins allow users and developers a website’s functionality beyond the features that come with a base WordPress installation.
What is a Widget? Widgets allow drag and drop sidebar content placement and assists with plugin implementation and extensions.