Telnet is a network protocol used to provide a command-line interface for remote control of computers and network devices over a TCP/IP network. It allows users to log into another device and execute commands as if they were physically present at the machine. However, Telnet transmits all data, including usernames and passwords, in plain text, making it highly vulnerable to interception and eavesdropping. Due to these significant security risks, Telnet has largely been replaced by more secure protocols such as SSH (Secure Shell) for remote management tasks.