HTTP is a client-server protocol

Most of the time the user-agent is a Web browser

a robot that crawls the Web to populate and maintain a search engine index

Each individual request is sent to a server, which handles it and provides an answer called the response

In reality, there are more computers between a browser and the server handling the request: there are routers, modems, and more

The user-agent is any tool that acts on behalf of the user

The browser is always the entity initiating the request

To display a Web page, the browser sends an original request to fetch the HTML document that represents the page. It then parses this file, making additional requests corresponding to execution scripts, layout information (CSS) to display, and sub-resources contained within the page (usually images and videos). The Web browser then combines these resources to present the complete document, the Web page. Scripts executed by the browser can fetch more resources in later phases and the browser updates the Web page accordingly.

A Web page is a hypertext document.

Between the Web browser and the server, numerous computers and machines relay the HTTP messages.

Those operating at the application layers are generally called proxies

transparent

non-transparent

Proxies may perform numerous function

caching (the cache can be public or private, like the browser cache)
filtering (like an antivirus scan or parental controls)
load balancing (to allow multiple servers to serve different requests)
authentication (to control access to different resources)
logging (allowing the storage of historical information)