Like Telnet, SSH is a network service that allows users to log in and gain command-line access to the server. Unlike Telnet however, SSH is considered secure because the data transferred between an SSH client and the server is encrypted. In fact, SSH stands for Secure SHell.
Only users with a valid shell can log in via SSH. For security reasons, the root user cannot log in via SSH by default, although this and many other SSH related options can be overridden by editing the SSH configuration file. If you need to do something as the root user on a Freedom 400x or above, you should first login as your administrative user, and then become root with the su command.
SSHd is the name of the server application that provides the SSH service. It is a stand-alone application that constantly listens for incoming SSH connections. It provides support for both the SSH1 and SSH2 protocols.