A signal is basically a message sent from one process to another, indicating a change of state.
Different versions of Unix use different names and/or numbers for some of the lesser used signals. See the signal(7) man-page. You can find the signals and numbers used in the POSIX standard, in /usr/include/bits/signum.h on a Linux system, or by invoking kill -l.
These hotkeys can be found by typing:
intr = [SIGINT] quit = [SIGQUIT] susp = [SIGSTOP]
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CTRL-S and CTRL-Q are used to control the flow of output to the terminal. CTRL-S tells the system not to send any more data to the screen until a CTRL-Q is pressed. You can use these to control fast scrolling output.