These are all GS1 events. If you are interested in working for other servers, I suggest you learn GS2 since Graal doesn't support GS1 anymore.
Triggeraction can be used in GS2, and you're correct in thinking it can be used to trigger another NPC.
For example
Lets say you had 2 NPC's in a level. One NPC is at 30, 39 and me NPC is at 40, 41. You can use Triggeraction to trigger the NPC at 40, 41 from the NPC at 30, 39 by doing something like this in the NPC script.
PHP Code:
triggeraction(40, 41, "triggerTest", NULL);
This will call the triggerTest action in the NPC at 40, 41. You can configure the triggerTest action in the NPC at 40, 41 by outing something along the lines of this in the code.
PHP Code:
function onActiontriggerTest() {
this.chat = "I've been triggered!";
}