Human can sense any things using their eye , ear,skin ,nose and toungue and they are processed and detected by brain But in the case of Arduino or any electronics device they donot have any such things as human they only understand electronics language . So we need to convert physical quantity or mechanical quantity that are exist in nature into the equivalent current or voltage . Those device which helps to react physical, chemical or biological condition is called sensors. For example a temperature sensor sense temperature and send equivalent current or voltage or pulsating clock to the microcontroller . According to this form the microcontroller do necessary job .Below is the block diagram of temperature sensor interfaced with microcontroller
Sensors and transducers are similar components , They both are actually used in sensing the physical ,environmental or biological condition . But the transducer indicates more than just sensors . sensor convert any condition into electrical form but the transducer may not convert in electrical form . Transducer are the device that are used to exchange energy into one form into another suitable form that can be measurable . If you are confused about these two quantity let us describe these terms using the following block diagram . Output from the sensor are analog in nature , we need to convert them into digital to fed into microcontroller . So an analog to digital conversion is necessary for the meaningful information .
Here first analog value from the sensor is read using the function analogRead(tempPin); and then these value read from sensor is converted into accurate numeric value and then these calculated value is displayed on the screen .