SSH (Secure SHell) is a protocol used to provide secure connections between two hosts. The secure connection is provided by encrypting the entire session between the two systems.
Each system has a pair of keys: a private and a public key. The keys are created at the same time, so they do have a direct relationship between each other.
The public key is made available to anyone who requests it. It is used to encrypt messages to be sent to its original owner.
The private key is the only key able to decrypt messages produced by its own public key. Needless to say, the private key may only be seen/read by its own owner.
The result is that any system wanting to communicate with your system must have your public key. And your system must have the public key of any system you wish to contact.
This allows two systems to have private conversations, even if someone is eavesdropping on their conversation. SSH is installed on all systems maintained by the Research Computing Support Group. In fact, as of May 2001, any session requiring a password with any UNIX system maintained by the Group will be required to be done using the SSH protocol. This includes file transfers and reading mail using remote clients.