"Signals are asynchronous events that can occur to a
running process and may be caused by hardware, software or users.
Signals are numeric integer messages that have been predefined so
they understand what these signals mean. When a process receives a
signal, that process must respond to the signal. Uncaught signals
will cause default actions to take place, which often means the
process is terminated. If you use "kill -l", or "trap -l" you can
get a list of available signals.