An example:

Below I am using the netcat telnet app by mudge of l0pht to connect to www.goolge.com on port 80. I want to get the root document (or /) so I GET /. the HTTP/1.1 specifies the version of the HTTP language that I wish to use. HTTP/1.1 format includes a host name so as to support virtual servers (websites that do not have their own IP address but share on with other sites). I finish up with 2 carrige return line feeds (Returns) to tell the server that I have finished requesting data.

Code:
E:\WINNT>nc www.google.com 80
GET / HTTP/1.1
HOST:www.google.com

HTTP/1.1 200 OK
Date: Thu, 10 Oct 2002 20:50:29 GMT
Content-Length: 2532
Content-Type: text/html
Cache-Control: private
Server: GWS/2.0
..Truncated...
Google sends back HTML (hypertext markup language) that is converted by your browser into what you see on screen.

If you can get to a web page via a web browser without a password you can also get to it via Telnet.

If you want to include passwords into the connection string below the this is done by converting it into Base64 and appending it to the url in the format user:pass@host. You can also use this method to post data to forms, send and recieve email, even IRC if you know the syntax of the language (eg HTTP SMTP POP3 etc). All of the languages spoken on the net are documented here.

Happy research! Hope that helps =]