Overview#Hypertext Transfer Protocol (HTTP) is a stateless request/response protocol that operates by exchanging messages across a reliable transport- or session-layer "connection".
The term "user-agent" refers to any of the various client programs that initiate a request, including (but not limited to) browsers, spiders (web-based robots), command-line tools, custom applications, and mobile apps.
The term "origin server" refers to the program that can originate authoritative responses for a given target resource.
The terms "sender" and "recipient" refer to any implementation that sends or receives a given message, respectively.
Messages are passed in a format similar to that used by Internet Mail RFC 5322 and the Multipurpose Internet Mail Extensions (MIME) RFC 2045 (see Appendix A of RFC 7231 for the differences between HTTP and MIME messages).