An HTTP request is a message sent from a client to a server to request a specific resource or action. It is initiated when the client sends an HTTP method (such as GET, POST, PUT, DELETE) along with a URL and any required headers or parameters to the server. The server then performs the requested action and sends back a response to the client. HTTP requests are a critical component of the World Wide Web, as they allow clients to access and interact with web resources across the internet.