A sensor is the point where the physical world enters an IoT system. It responds to a quantity, such as temperature, pressure, humidity, acceleration or gas concentration, and produces a signal. Analog sensors output a voltage or current that an ADC turns into numbers; digital sensors contain their own converter and talk to the microcontroller over a bus such as I2C.
Sensor choice shapes everything downstream. Accuracy, drift over time, response speed and power draw all have a cost, and a cheap sensor that reads a few degrees off can make an entire cold-chain dataset useless. Sampling less often saves energy but can miss short events, which is a recurring trade-off in battery-powered designs. For how sensor readings travel to the cloud and back, see How IoT works.
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