Secure Shell or SSH is used primarily for accessing a remote system, but it has many more features than that.
Shell Access File transfers Executing remote command Port forwarding and tunneling Creating VPN Forwarding X displays Encrypted proxy browsing (via SOCKS) Mounting remote directories Execute commands
ssh user@hostname [command]
Authentication Mechanisms
Password Public-key Keyboard-interactive Multiple passwords required (SecurID, etc.) GSSAPI (Generic Security Services Application Program Interface) Extensible to provide for Kerberos, NTLM, etc. There is a great resource out there that gives examples of some of what the features listed above can do.