You have asked a lot of questions, and reading on Wikipedia will probably help you most.
NPN and PNP are both junction transistors. They refer to the types of semiconductor used. Your schematic shows a NPN transistor, and you appear to be using it correctly.
A positive voltage applied to the base relative to the emitter will allow a current to flow from the (positive) collector to the emitter.
(This statement is a simplification).
For a PNP transistor, the statement above reads:
A negative voltage applied to the base relative to the emitter will allow a current to flow from the (negative) collector to the emitter.
I.e. they switch and control current in a different way. Your NPN is what is know as a "high side switch". You could have a different schematic and use a PNP but to control it with your Arduino is would have to be a "low side switch". Depending on what you want to control and the characteristics drives what type to use. Both types can have very different ratings.