An NPN switches a load to ground. A PNP switches a load to V+. Same three pins, same general topology, but everything is inverted: arrow direction, base polarity, current direction. The first time you build one, the inverted logic feels weird; by the third one it just feels like the mirror image it is.
What "high-side" means and why it matters
In the NPN low-side switch, the load is between V+ and the collector — your motor is permanently connected to the +12 V supply, and the transistor decides whether ground is reachable. That works, but it leaves the load's high-voltage side always energized, which is awkward for safety (touching a connected-but-off device's positive terminal still gives you 12 V) and for any control circuit that expects to see the load floating when off.
The PNP high-side switch flips this: the load sits between the collector and ground; V+ goes to the emitter. The transistor decides whether the load even sees V+. When the transistor is off, the load is electrically disconnected from the supply. That's how lighting circuits, automotive ECUs, and many industrial controllers prefer to switch — high-side, so a fault on the load side doesn't keep current flowing.
The polarity flip
An NPN turns on when its base voltage exceeds its emitter voltage by ~0.7 V. With the emitter at ground, that means base > 0.7 V — a HIGH logic pin works directly.
A PNP turns on when its emitter voltage exceeds its base voltage by ~0.7 V. With the emitter at 5 V, the base needs to be pulled BELOW ~4.3 V. A LOW logic pin (0 V) accomplishes this easily — current flows OUT of the base, through the resistor, into the chip pin. D2 LOW turns the LED on. The Arduino sketch reads inverted from a beginner's intuition.
Choosing between NPN and PNP
For toy projects: NPN is almost always simpler. The driver chip's HIGH = load ON mapping is what every other circuit you'll meet looks like.
For systems where the load must be galvanically disconnected when off: high-side PNP (or its modern silicon cousin, a P-MOSFET). For low-voltage, low-current LED switching, the difference is academic. For 12 V automotive loads, getting it right is the difference between a clean shutdown and a corroded fuse box.