It's not really that difficult...
I believe with parallel, each point recieves the same voltage, and if one is removed, the others are not affected. but in a series circuit, all points rely on the one before it, and as the voltage travels down the circuit, the total voltage decreases, and if one is rmeoved, they all fail..
So like, in a trigger mod using two leds, if you use a series circuit, one led would be brighter than the other..
It's been awhile since I learned circuits, but I am pretty sure that is at least close to correct >.> someone please correct me if I am wrong.