OpenSSH is a standard part of most distibutions. By providing a command line shell into the host it provides the same level of access to a normally booted host that physically being at the machine would. The server portion runs on each host and requires a firewall be opened to allow remote users to contact. The default for most modern distributions is to install and run this ssh server portion on each host. The ssh client, run by the user then connects and does authentication. All of this happens via an encrypted link. This allows for secure communcation between two remote sources over untrusted networks.