Secure shell or SSH is both a program and a network protocol for logging into and executing commands on a remote computer. It is intended to replace rlogin, telnet and rsh, and provides secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.
The program is a common Unix shell program, but there exists implementations for most modern platforms, including Microsoft Windows (where one of the most popular is PuTTY) and Mac OS. There are commercial versions, freeware versions, and open source versions.
A later version of the protocol was released under the name SSH2.
OpenSSH is an open source implementation of SSH. Corkscrew is a tool enabling the user to run SSH over HTTPS proxy servers.
GNU has its own implementation of SSH, lsh.
An IETF working group, secsh, is currently in the process of standardizing the protocol.
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This article (or an earlier version of it) contains material from FOLDOC, used with permission.