client
      A program that establishes connections for the purpose of sending
      requests.

   user agent
      The client which initiates a request. These are often browsers,
      editors, spiders (web-traversing robots), or other end user tools.

   server
      An application program that accepts connections in order to
      service requests by sending back responses. Any given program may
      be capable of being both a client and a server; our use of these
      terms refers only to the role being performed by the program for a
      particular connection, rather than to the program's capabilities
      in general. Likewise, any server may act as an origin server,
      proxy, gateway, or tunnel, switching behavior based on the nature
      of each request.

   proxy
      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.