kill -9 is actually shorthand for kill -SIGKILL
The unix kill command, named for its most common use, is really a more general purpose tool for sending signals. These are commonly referred to via symbolic name, and then abbreviated for convenience while typing. 9 for SIGKILL, 1 for SIGHUP, etc. A number of these signals have been documented here via their symbolic names.
All signals available under the kill command can usually be determined by typing
The most commonly known signal and by far the most frequest use of the unix kill command, SIGKILL is indeed the most feared and respected signal in the arsenal. However, it is not 100% foolproof.
Inexperienced users and administrators take note: overuse of SIGKILL can create problems, as many programs do cleanup operations in signal handlers for less drastic signals. This command should be used as a last resort, after an ordinary