Most outdoor light sensors operate by detecting heat (called infra-red in technical speak). This will ensure that only people (and animals) set off the sensor.

I am not sure if he is referring to PIR sensors - he might be referring to sensors which detect light (ie tell the difference between night and day).

These type of sensors change resistance (photoconductive sensor) or voltage (photovoltaic sensor) depending upon how much light is hitting the device.

I think he is referring to PIR sensors which standards for "passive infra red". These are triggered when a certain level of heat (infra red) is measured by the sensor's receiver.

Motion sensors are sensitive to the temperature in the range that human beings produce.

It is actually more complex than just infrared detection.

Most lights will not go off when someone is not moving. That's because the sensor is looking for a rapid CHANGE in heat (infrared) which is caused when the person moves in front of the sensor.