Secure Shell (SSH) is a protocol that allows for secure communication between two devices. It's often used to log into remote servers and to execute commands, as it provides a more secure authentication process than standard protocols like Telnet or FTP. SSH is also used to create tunnels for securely transferring data between two devices.
It uses public-key cryptography to create a secure, encrypted connection. SSH is widely used to access remote systems and to transfer files securely.
The SSH protocol is defined in RFC 4253. It is based on the earlier Telnet protocol, but provides many additional features that make it more secure. SSH clients are available for most major platforms, including Windows, Mac OS X, and Linux. Servers can also be installed on most platforms.
One of the most popular SSH clients is PuTTY, which is available for Windows and MacOS.