SSH, or Secure Shell, is a cryptographic network protocol used for secure data communication, remote command-line login, and other secure network services. It provides a secure channel over an unsecured network by using encryption, ensuring that the data transmitted between the client and server remains confidential and integral. Primarily used by system administrators and developers, SSH allows users to access remote machines securely. It replaces older protocols like Telnet and rlogin, which transmit data in plain text, making them vulnerable to eavesdropping. With SSH, users can execute commands, transfer files, and manage systems securely over the internet.