HTTP was designed from the beginning to support multiple representations of the same resource. A user profile can be represented as JSON for a machine consumer or as HTML for a browser. A dataset can be returned as JSON or as CSV depending on what the client intends to do with it. An image can be served as JPEG or WebP based on what the browser supports. Content negotiation is the mechanism through which clients and servers agree on which representation to use — and understanding it is essential for building APIs that serve diverse consumers cleanly.