Ssh (Secure Shell) is a means for logging into remote hosts via an encrypted connection. Because of the encryption a greater security is obtained in the the login process because a user's password is never transmitted in cleartext form across the network.
For those people familiar with rlogin,rsh, and rcp, the unix ssh suite of applications can be used in place of these utilities with identical command-line syntax. The corresponding ssh utilities are slogin, ssh, and scp (although slogin is just a symbolic link to ssh).
Ssh make use of public key cryptography for both authentication and connection establishment. It makes use of the RSA public-key algorithm. For more information on this, see RSA Laboratories page on public-key cryptography and the RSA algorithm.
Ssh can make use of several cipher algorithms for encrypting transmission. These include IDEA, Blowfish, 3DES, and DES. The default is IDEA. The weakest (and not recommended) is DES.
The primary SSH homepage is here.