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Definition
HTTP (Hypertext Transfer Protocol) is the foundation protocol of the World Wide Web. It defines how messages are formatted and transmitted between web browsers and servers, using a request-response model where clients send requests and servers return responses.
Every web page load starts with an HTTP request. The browser sends a request line (method + URL + version), headers (metadata like User-Agent, Accept, cookies), and optionally a body (for POST/PUT requests). The server processes the request and returns a status line (like "200 OK" or "404 Not Found"), response headers (content type, caching directives, cookies), and a body (the HTML, JSON, image, or other content).
GET retrieves data without side effects — it is safe and idempotent. POST submits data to create a resource. PUT replaces an existing resource entirely. PATCH partially updates a resource. DELETE removes a resource. HEAD is like GET but returns only headers (useful for checking if a resource exists). OPTIONS describes which methods an endpoint supports, used in CORS preflight requests.
HTTP/1.1 (1997) added persistent connections and chunked transfer encoding. HTTP/2 (2015) introduced multiplexing (multiple requests over a single connection), header compression, and server push. HTTP/3 (2022) replaces TCP with QUIC (based on UDP), eliminating head-of-line blocking and reducing connection setup time. Modern browsers and servers support all three versions, negotiating the best one automatically.