Telnet is a protocol for remotely connecting to a machine and getting a console (a CommandLine) on that machine, as if you were local.
It was invented for ArpaNet and used extensively on the InterNet.
See telnet(1) for information on the telnet client in Linux.
Telnet is totally insecure. (all plain text; no encryption.) While there exists a Telnet over SSL, it has been replaced almost entirely by SSH.
