With UNIX, there is no need to be in front of a computer to start a session with it. One only needs to connect to the computer using a UNIX connection command.

Historically, there exists two connection commands: telnet and rlogin. They take as arguments the computer name and the login name.

rlogin machine.subdomain.dom -l user
telnet machine.subdomain.dom

Then, the password is asked.

Unfortunately, these commands are by design not very much secure (=the password is transmitted through the network without encryption)

It is therefore very much advised to use a secure connection command: ssh (secure shell).

With ssh, commands can be directly executed on a remote host:

ssh user@machine.subdomain.dom [command]

Without command, ssh will connect to the remote computer and start a new interactive shell.