An excerpt from the ssh readme file
SSH (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. It provides strong authentication and secure communications over insecure channels. It is intended as a replacement for rlogin, rsh, rcp, and rdist.
WHY TO USE SECURE SHELL
Currently, almost all communications in computer networks are done without encryption. As a consequence, anyone who has access to any machine connected to the network can listen in on any communication. This is being done by hackers, curious administrators, employers, criminals, industrial spies, and governments. Some networks leak off enough electromagnetic radiation that data may be captured even from a distance.
When you log in, your password goes in the network in plain text. Thus, any listener can then use your account to do any evil he likes. Many incidents have been encountered worldwide where crackers have started programs on workstations without the owners knowledge just to listen to the network and collect passwords. Programs for doing this are available on the Internet, or can be built by a competent programmer in a few hours.
SSH is available for almost all Unix platforms, and commercial versions are available for Windows (3.1, 95, NT) and Macintosh. For more information, see http://www.datafellows.com/f-secure
OVERVIEW OF SECURE SHELL
The fiqrst form opens a new shell on the remote machine (after authentication). The latter form executes the command on the remote machine.
When started, the ssh connects sshd on the server machine, verifies that the server machine really is the machine it wanted to connect, exchanges encryption keys (in a manner which prevents an outside listener from getting the keys), performs authentication using .rhosts and /etc/hosts.equiv, RSA authentication, or conventional password based authentication. The server then (normally) allocates a pseudo-terminal and starts an interactive shell or user program.
For more information, see the manual pages
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