SSH stands for ``Secure Shell,'' a secure replacement for rsh and rlogin created by SSH Communication Security Ltd. (http://www.ssh.com/). SSH uses proven encryption algorithms to provide a secure alternative to the aforementioned rsh suite, as well as telnet and ftp.
Using SSH is not difficult, but it can take time to absorb what exactly SSH is doing behind the scenes, and how to take advantage of the services it provides. There are two versions of SSH (named, appropriately enough, SSH1 and SSH2) which operate in much the same way, although there are subtle differences in their configurations which will be outlined here.