Back in the old days, if you wanted to establish an interactive login session over the network, you used telnet or rsh. However, as networking became more popular, these tools became less and less appropriate, because they're horrendously insecure.

The data going between the telnet client and server isn't encrypted, and can thus be read by anyone snooping the network. Not only that, but authentication (the sending of your password to the server) is performed in plain text, making it a trivial matter for someone capturing your network data to get instant access to your password. In fact, using a network sniffer, it's possible for someone to reconstruct your entire telnet session, seeing everything on the screen that you saw!

Obviously, these tools that were designed with the assumption that the network was secure and unsniffable are inappropriate for today's distributed and public networks.