SSH (Secure Shell) is a secure alternative to the ancient Telnet program/protocol. Telnet (and SSH) allows a user to connect to a remote server, and enables the users to use a command line interface to execute commands (manage the server).

Where Telnet is relatively limited in its functionality, SSH has a bunch of features which enables the user to do much more. The SSH protocol has the possibility to tunnel traffic through an SSH connection (read: tunnel). The big advantage is that everything going through the tunnel is heavily encrypted (which is good).

The tool best known to use SSH is SFTP (FTP over SSH). A secure alternative of the 'old' (in plaintext communicating) File Transfer protocol.