SSH (secure shell) is an interesting protocol. It is a TCP/IP based protocol, it is server-client based, and it can be used for many purposes. Furthermore, all traffic is encrypted, and all connections require an initial authentication.
Okay, so what can we do with SSH?
SSH can replace telnet, as it can be used to remotely open a command line interface to another computer. Note that the remotely controled host is the server, and the controling host is the client.
SSH can replace ftp, as it can be used to transfer files.
What else can SSH do? It can forward ports. This allows all kinds of protocols to be tunneled through a single port 22 (22 is the default port, but other ports can be used. We’ll get to this later.