Help understanding NPN & PNP Sensors

All I know is that and NPN & PNP are 3 or 4 wire sensors.  It can be capactive or inductive.  I know for a 3 wire sensor it's color are Brown (+), Blue (-), and Black (signal).  A 4 wire sensor colors are the same as the 3 wire sensor, but also has a white (signal) wire as well.

I keep hearing "switches on the high side (sourcing)" or "siwtchies on the low side (sinking)."  What do they mean by that?  That's what throws me off completely.  What is the purpose of NPN & PNP?  Why not just have A sensor?!

The transistor isolates the sensor from the cabling going to the processor, and it allows the resitive load for the transistor switch to be at the far end of the cable at the processor, which means that a pull up or pull down resistor can be left in place in the processor to leave the input at a known state when no sensor is connected.
A simple explanation of the difference is here,
http://uk.rs-online.com/web/generalDisplay.ht...infozone&file=automation/sensing-technologies
cheers,
Richard