**Used to securly connect to a remote server** using Diffie-Hellman algorythm for key exchange, ssh replaces the archaic telnet program for remote shell sessions.
If you wish to use a different username then the one initiating the session (i.e. you're using ssh from root but wish to log in as different user to a remote system), the form `ssh username@remote_address` can be used, which will then prompt for the user password on successful connection.