To exit from less, you can press 'q'. Other applications may have a different terminator, depending on the fact they expect an EOF (Ctrl-D) or a termination signal (Ctrl-C). For example you can use Ctrl-D to send the "end of input" signal when using
other commands don't receive input from the terminal and you will stop them using Ctrl-C. For exemple when using
Another signal is Ctrl-Z that means suspend. This is useful when you launch a command from the terminal and you want to put it in background, for example
$ # prompt returns here and the job is suspended
$ bg # this put the last (previous) job in background.
Hope this helps.