set the exitLock = truewill disable automatic quitting with command-Q or command-. on the Mac. In Windows it prevents quitting with ctrl-Q, but not quitting with Esc; nor will it disable process-switching via Alt-Tab, making it a pretty useless feature on the PC (how many Windows users are gonna reach for ctrl-Q to quit in the first place?).
By convention, disabling of command-Q should generally be restricted to kiosks and similar applications where you want to prevent the public from getting at the host machine. For consumer products, preventing people from quitting by the standard means tends to only piss them off.
The exitlock does provide a useful way of trapping direct quitting if you need to do additional cleaning up first, but you should still quit once that's taken care of (listen to me, I sound like the Apple Thought Police). To trap for command-Q you can use the exitlock in conjuction with a keyDown script like this:
on keyDown
if the key = "Q" and the commandDown then
cleanMeUp
quit
end if
end keyDown
The exitlock property has no effect when running in the Director application
itself.