Developed by SSH Communications Security Ltd, Secure Shell is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another using security methods. It provides strong authentication and secure communications over insecure channels. It is a replacement for rlogin, rsh, rcp, telnet and rdist.
SSH protects a network from attacks such as IP spoofing, IP source routing, and DNS spoofing. An attacker who has managed to take over a network can only force ssh to disconnect. He or she cannot play back the traffic or hijack the connection when encryption is enabled.
When using ssh's slogin (instead of rlogin) the entire login session, including transmission of password, is encrypted; therefore it is almost impossible for an outsider to collect passwords.
Nowadays a lot of cloud providers provide ssh free of charge within their cloud environment.