What exactly is a "Sensor" I know this questions sounds kind of stupid but I don't get it. 

My question is "What is a sensor?" The answer is obvious


  "In the broadest definition, a sensor is an object whose purpose is to detect events or changes in its environment, and then provide a corresponding Output"


Source: Wikipedia

While making my research about sensors I realized that this definition
doesn't work for Smartphones or they just don't care. 

If you search for some sensors they list you something like Accelerometer, Light Sensor and so on but they don't list something like Bluetooth, Wifi, Microphon, Camera... 

But they are sensors (or use sensors) as well. Why don't they list them in a sensor overview?
Tricky question... But the answer is quite simple. It's the purpose of the sensor. If it's built to detect (or sense) changes of things (like gravity or light) and give an output at the moment it detects a change it's a sensor.

If it's built to connect to other devices or to take pictures when you click a button it's a sensor but it's not considered to be a sensor by most people and manufacturers.

Let's take a camera as example. In your smartphone the camera is only running when you start it that's why it's not considered to be a sensor even though a camera uses a CMOS sensor. Simply because its normal operating state is turned off and not to detect changes.