What is a temperature sensor?

Let's start from the basics... discussing what a temperature sensor is: 

As the name indicates, a temperature sensor is an instrument that can be used to measure temperature. It has an output signal proportional to the applied temperature. When the temperature of the sensor changes, the output will also change accordingly.

There are various kinds of temperature sensors that have different output signals. Some have a resistance output, some have a voltage signal, some have a digital signal and many more.

In practice, in industrial applications, the signal from temperature sensor is typically connected to a temperature transmitter, that will convert the signal into a format that is easier to transfer for longer distances, to the control system (DCS, SCADA). The standard 4 to 20 mA signal has been used for decades, as a current signal can be transferred longer distances and the current does not change even if there is some resistance along the wires. Nowadays transmitters with digital signals or even wireless signals are being adopted.

Anyhow, to measure temperature, the measuring element that is used is the temperature sensor.