Telnet is a protocol used on remote web servers that allows a remote user to access the operating system on that remote machine. It sets up a virtual terminal with the remote computer using Transmission Control Protocol (TCP). The process of using Telnet to connect to a remote web server is often known as “telnetting in” to the machine.
A typical use of Telnet might be to log in and change a password on the server. One can usually run most Unix style commands using Telnet to call up directory and file listings for example. There could also be a computer program on the remote machine that has no web interface which would require a user to use Telnet and execute the command.
Telnet often refers interchangeably with the protocol itself and the program that uses the protocol. Telnet actually dates back prior to 1973 before a standard protocol was even created so it predates TCP/IP which it is tied to in modern times. Almost every computer platform in the world provides for Telnet access.