Telnet is an early network protocol that allows text-based access to remote systems but lacks security features. It works at the application layer and provides bidirectional interactive text communication through a virtual terminal connection. SSH was developed as a secure replacement for Telnet, supporting encryption, authentication, and integrity to prevent eavesdropping and spoofing. It uses public/private key cryptography to securely transmit data and authenticate systems. While still used occasionally, SSH is now generally preferred over Telnet for remote access due to its enhanced security.