The Hypertext Transfer Protocol specifies how a client machine requests
information or actions from servers. This protocol specifies how two machines share information, which is called a request. These requests are composed of several
parts which I’ll outline below.

The first line of an HTTP request is called the
Request-Line.
It contains:

Let’s take a closer look at these four elements.

URI

A URI or Uniform Resource Identifier is how objects are identified. Clients use URIs to tell the server what object
to act on for a given request. In more general terms a URI is nothing more than a web address.

Read more at the source