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 = ^C; quit = ^\; kill = ^U; susp = ^Z
intr = [SIGINT] quit = [SIGQUIT] kill = [SIGKILL] susp = [SIGSTOP]
Here are a couple of pages that show how to code for signals:
http://users.actcom.co.il/choo/lupg/tutorials/signals/signals-programming.html
http://www.gnu.org/software/libc/manual/html_node/Signal-Handling.html
5 pages link to Signal: