Telnet

Telnet is a network protocol used on the Internet or local area networks (LANs) to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection.

Through Telnet, an administrator or another user can access someone else's computer remotely.

On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer.  With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer.

Most network equipment and operating systems with a TCP/IP stack support a Telnet service for remote configuration.  However, because of serious security issues when using Telnet over an open network such as the Internet, its use for this purpose has waned significantly in favor of SSH.