resistors and led's

Sup 12V? I have a question. I am familiar with the setup of LED's and wiring them to a 12V auto battery. The normal way is to apply a resistor to each LED to lower the voltage to approximately 3-5V. No problem. But what strikes my curiosity is... why is it not okay to place the resistor before a SERIES of LED's? Would it not have the same results (dropping voltage on one line) as applying one resistor per LED? I've heard people say it isn't good, but never gave a reason why. I would think the line would have the correct voltage after the resistor and that would suffice in wiring as many LED's after it as amperage allows. Would anybody explain why it's "not a good idea"?

^^^ The question was merely asking for information.... which you've just done. I think that answers it.. I need to do more research on resistors themselves. Thanks for your input. I know resistors are cheap and that's how I'll wire them.. in parallel rather than series. I was just misinformed on how resistors worked exactly. Positive rep to you