Servers send information. Clients retrieve. Simple.
Windows comes with a built in program to connect to other computers called telnet.
To start Windows telnet Start menu> Run> type Telnet. Click connect> remote system
Ports are doors into computers. Hosts are computer names
(ip number or a name that is translated into the ip automatically)
Different programs open different ports, but they always open the same ports so other computers know which port to connect to. You can get a port list listing all the different ports, but a basic one is:
11 :- Sends info on the computer
21 :- FTP (File transfer program)
23 :- Telnet (Login to the computers command line)
25 :- Smtp (Sends mail)
80 :- Http (Web pages)
There are thousands of different programs using different ports. You can get programs called portscanners which check a computer for all ports up to a certain number, looking for ways in. You can portscan a computer looking for ways-in.
Anyway, back to telnet.
Type www.yahoo.com as the host and port as 80 the click connect.
If nothing happens, you're in. Wow. You are connected to Yahoo's server.
You can now type http commands (you are connected to an http server, so it supports http commands). Ie. on an ftp server you can type open and it will do something. On an http server it will just wonder what the hell you are on about.
Type get / http/1.0 then press enter twice to get the file on the server at / (try /index.html) etc.)