Wall mounted sensor/PIR switches offer greater control vs regular on/off switches over the activation of an electrical circuit such as the length of time it is on, the brightness level it activates at etc.
When motion is detected inside a room (either by infra-red, microwaves etc), the switch will activate, allowing for lighting systems or other electrical circuits to switch on. A sensor switch is a good solution for controlling the energy usage of a space, reducing your carbon footprint.
PIR switches are a great solution for replacing traditional on/off light switches. They often provide features allowing control of the length of time the light is on, the detection light level and range settings, as well as providing energy savings by only activating when a room is in use.
Sensor switches can be prone to a variety of false or unintended activations, pets and various animals including insects can activate them. It is also possible for a falling object to activate certain types of sensors, however this is likely quite rare.