What is the difference between npn and pnp transistor?

In NPN transistor, Base is connected to positive terminal of battery.

Now, when current is switched on, Base-Emitter junction (indicated in image) is forward biased, whereas Base-Collector junction is reverse biased.
Remember that forward bias narrows depletion region, whereas reverse bias widens depletion region.

n-type region(lower one) on emitter side has majority electrons (reason 'n' stands for negative).
These electrons move from emitter side to base.

Base is thin region, which has holes. When electrons move from emitter to base only a few electrons combine with holes. Whereas others pass from base to collector.

In collector the electrons are pulled by positive terminal of battery(right hand side battery). And thus the cycle of current continues to flow.

That is whole working of NPN transistor.