Answer:
"P" is for Positive and "N" is for Negative
So basically put a PNP Transistor Would use N to Switch P, in the name "PNP" or "NPN" the first character is for the polarity of the Collector-pin, the second for the Base-Pin, and the third for the Emmiter-pin.
So if you have a PNP Transistor you can`t just replace it with an NPN as the polarities differ. If you can find a way to change those polarities then sure it could work.
The Collector-pin basically receives the bigger current. The Base-pin determines how much of that current will be transferred to the emmiter-pin. So in a PNP the base current could for example be 0V and the Collector 5V, this will allow a free flow of current from Collector to Emmiter, the usage of a transistor in many cases is to switch high current with lower current.
The main difference is that a PNP transistor uses "holes" as carriers and an NPN transistor uses electrons as carriers (It is to be remembered that the flow of current is always in the direction opposite to that of the flow of electrons).
The difference in the symbol for the two transistors are that the PNP transistor will have an arrow pointed to the base from the emitter, and the NPN has it pointing outside.