Battery Pack Building.....series/parallel???

This is kind of follwoing on form the first battery pack thread - however I'm hoping someone can explain to me the differences in wiring batteries in series and in parrallel.

I'll be using one of the prefab packs available and slotting in 4x 18650s. But if I'm running a simple flashlight - maybe 2 form this pack should I set them up in parrallel and maintain the 3.7v or in series and achieve 14.8v. I understand that 14.8 may blow certain drivers or LEDS.

In short what are the pros and cons of parrallel and series connecting?
and what P2PS etc etc mean?

One more question - when connecting leds to drivers and batteries etc - what thickness/type of wire does everyone use?

Thanks for your help.

J

What voltage does your design require?

I guess I'm answering my own question here - but its currently running off 1x 18650 cell.

Would I be right in thinking I need to connect in parrallel to maintain this voltage for fear of blowing everythiing up?

However I do have some LEDS that can take upto 18volts - would I be better off using 14.8v in series or 3.7v x4 in parrallel?

Or am I over simplifiying a rather complicated situation?