What is HTTP?
The application protocol the web is built on, a simple request/response format for asking a server for a resource.
HTTP (HyperText Transfer Protocol) is the protocol every browser, mobile app, and API client uses to talk to web servers. The client sends a request line ("GET /index.html HTTP/1.1"), some headers, and an optional body. The server responds with a status code (200, 404, 500…), headers, and a body. That's essentially the whole protocol, everything else is layered on top.
HTTP/1.1 (1997) is plain text and one request per connection at a time. HTTP/2 (2015) added multiplexing, header compression, and binary framing, multiple requests per connection without head-of-line blocking. HTTP/3 (2022) ditches TCP entirely and runs over QUIC for faster connection setup and better mobile performance.
Understanding HTTP at the wire level is non-negotiable if you build anything web-facing. Every API design decision, every caching question, and every weird production bug eventually comes back to status codes, headers, and the lifecycle of a single request.
In the wild
GET /api/users/42 HTTP/1.1, fetch user 42POST /sessions HTTP/1.1with form body, log in- A
Cache-Control: max-age=3600header, cache for an hour