Years ago, the standard way to log in to a remote computer in order to run programs on it was to use a protocol called Telnet. You opened a window in a Terminal emulator program, typed telnet followed by another computer's domain name or IP address, typed your credentials, and you were in (via a command-line interface). Telnet still exists, but it's fallen into disuse because it doesn't offer encryption and has numerous other security vulnerabilities that put your data (and, in fact, the entire remote computer) at too great a risk. The most common replacement is SSH, which also provides a command-line interface to a remote computer but encrypts the connection.
For more on enabling Remote Login on your Mac so that another computer can use SSH to log in to it, see Chapter 7.
In order to connect to a remote computer using SSH, you must have an account on that computer, and it must be running some variety of SSH software. If the other computer is a Mac, you can connect to it with SSH as long as it has Remote Login activated in the Sharing pane of System Preferences. Virtually all Unix computers have SSH capabilities built in, although it's less common on Windows PCs.
The simplest way to use SSH is to use password authentication, which (as the name suggests) means you type your username and password each time you log in, and the server checks them against its records to make sure you have permission ...