What is a Sensor and what is an Actuator?

A sensor is a device that can trigger something but can not do anything itself, like a switch, a thermostat, or a presence sensor. An actuator can do something like a light, a blind, or a heater.

A sensor can be connected with an actuator in the free@home SysAp (under Devices->Floorplan or similar, do not know exactly what the menu item is called in English GUI), a Switch can control light, a thermostat a heater, and so on. A sensor can also be used as input for actions to be more flexible (switch the light on when the door is open). An actuator can be also controlled from a web GUI, from APP, by schedules, or in actions.

The actuator can be found in two form factors, REG, which is built into your central installation, then you use sensors in your rooms only, or as a combination of an actuator and a sensor. By default, this combination of sensor and actuator is linked together, but that can be changed in the SysAp.

The Picture shows a Binary Sensor, you can connect different binary inputs to it (like Sensors from your windows), and can work in different modes, depending on the device you connect to it

https://manuals.busch-jaeger.de/Systems/Freeathome/EN/ABB/SETT/page_74.html