Temperature sensors (DS18B20) are digital thermometers that get temperature readings and output them in a digital format. They only need 1 digital pin from the Arduino and a specialized library to communicate with them.

Temperature Sensor Library

#include <OneWire.h>
#include <DallasTemperature.h>

OneWire oneWire(2);
DallasTemperature sensors(&oneWire);

void setup() {
  sensors.begin();
  Serial.begin(9600);
}

void loop() {
  sensors.requestTemperatures();

  float val = sensors.getTempCByIndex(0);

  val = sensors.getTempFByIndex(0);

  Serial.println(val);
}