Telnet allows you to submit commands over the network in plain text. It was used to execute shell commands on remote computers, but this usecase has been superceded by ssh which protects passwords by encrypting the communication.
For controlling a computer via the network use ssh, not telnet!!!
Telnet is still good for
Here is for example how you query the http port to test if it is working:
tweedleburg:~ # telnet localhost 80 Trying ::1... Connected to localhost. Escape character is '^]'. GET /index.htm HTTP/1.1 Host: localhost HTTP/1.1 200 OK Date: Sun, 03 Jul 2011 18:14:34 GMT Server: Apache/2.2.17 (Linux/SUSE) Last-Modified: Sun, 03 Jul 2011 18:13:44 GMT ETag: "576863-54-4a72e360d0600" Accept-Ranges: bytes Content-Length: 84 Content-Type: text/html <html> <head> <title> Test page </title> </head> <body> hello world </body> </html>