The outputs of some sensors will behave like transistors, when a sensor senses an object it will trigger the transistor controlling the output (which essentially acts like a switch) and depending on its design it’ll act as an NPN or PNP type transistor.  An NPN output is commonly called a “sinking” output.  When the sensor senses an object it will connect the output to the negative supply.  A PNP output is commonly called a “sourcing” output. When it senses an object it will connect the output to the positive supply.  If you’re unsure of which output type you need, a number of manufactures produce sensors with configurable outputs. In this case the sensor can be configured to operate as an NPN or PNP type.  NPN, PNP, and Configurable type sensors come in a Normally Closed and Normally Open configurations.