Most servers providing a service (such as websites and emails) over the modern internet aren't attached to keyboards or monitors. Even when servers do have local input/output hardware, remote access is often much more convenient.
Various protocols have been used to provide remote command-line access to servers. One of the first such protocols was Telnet. With Telnet, a client remotely connects to a server using plaintext over TCP port 23. The server provides more-or-less direct access to the operating system command-line through this Transmission Control Protocol (TCP) connection. The client sends plaintext commands to the server, and the server executes these commands. The command-line output is sent back from the server to the client.
Telnet has a major security shortcoming: it does not encrypt any data sent over the network. Even user passwords...
