Just complementing the previous answers:
'sudo halt' can be used to shutdown the computer (however, if it is a server,
you probably do not want to shutdown!);
Ctrl+C aborts the running command (see the manual of 'kill' to abort another
process, e.g., that runs in the background).
A command is stopped (so that you are back to the prompt) with Ctrl+Z. You
can run it back in the foreground with the 'fg' command (followed by the
number output by Ctrl+Z; use 'jobs' to have the ids of every task that was
stopped or runs in the background), in the background with 'bg';
By default, six text sessions are available. You can switch between them with
Ctrl+Alt+F1, Ctrl+Alt+F2, Ctrl+Alt+F3, etc.
You may like to read this free documentation for useful commands:
http://en.flossmanuals.net/command-line/
