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.
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: