Secure Shell (SSH) is an Internet communication protocol used mostly to allow users to log into other computers and run commands. It lets people exchange data using a secure channel between two computers. It is used mainly on Linux, Macintosh and Unix computers. It is a lot like Telnet, but is safer. It is less likely to be hacked as Telnet is.
SSH was designed by Tatu Ylönen in 1995 to replace the less secure protocols rlogin, Telnet and rsh, which were used to access shell accounts on remote computers.