The fundamental communication protocol for the functionality of the World Wide Web is HTTP (Hypertext Transfer Protocol). This protocol allows web browsers and servers to communicate by defining how messages are formatted and transmitted. When a user requests a web page, the browser sends an HTTP request, and the server responds with the requested content. Additionally, a secure version of HTTP, known as HTTPS, adds a layer of security through encryption, protecting the data exchanged between the user's browser and the server, which is crucial for transactions involving sensitive information.