Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. (Definition from Wikipedia)
This website gave a fascinating example of using Telnet to watch StarWars, the full movie!
How many interesting people are there doing IT!
Historically, Telnet provided access to a command-line interface on a remote host. However, because of serious security concerns when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH.