Hypertext Transfer Protocol (HTTP) is a a request/response application layer protocol. It is part of the internet protocol suite (TCP/IP). As the name suggests, it transfers hypertext (web pages). Although HTTP isn’t the only protocol in town, it is one of the most widely supported and used.
A computer browsing the internet (client) sends an HTTP request to the web (server). If the requested resource is available, the web server sends an HTTP response containing the requested hypertext. Basically, HTML and the associated CSS, JavaScript, etc. If the resource isn’t available, the server might respond with a predefined hypertext document known as a 404 page.
User agents (UA)
Obviously, the computer isn’t doing all of this by itself. There’s software involved. That software is called a user agent, or UA for short. A user agent is pretty much any software that accesses the internet. Probably the most common example of this is the web browser. But there are others. Web crawlers for search engines and mobile apps that access content from the web are other examples of user agents.
History of HTTP
The standards on which HTTP is based are developed by The World Wide Web Consortium and the Internet Engineering Task Force. Each update to the HTTP standards is published in a document known as a request for comment (RFC). The most recent update as of this writing is HTTP/2, described in RFC 7540. HTTP/2.0 replaces HTTP/1.1 (RFC 2068 & 2616), which replaced HTTP/1.0 (RFC 1945), which replaced HTTP 0.9.
See also: The difference between an ehternet splitter a hub and a switch