WordPress is a CMS or Content Management System so we will start there.
What is a CMS?
A CMS has the ability to store, organise and display documents, images video and any other online media. It can be edited by multiple users and is the foundation for many large websites like newspapers and government websites.
WordPress is a open source CMS website creation tool
It features architecture for plugins that add functionality to website and a theme template system for the visual aesthetics of the website.