PIR sensors (or passive infrared sensors) are motion sensors that measure subtle changes in infrared light in the room and register that change as movement. The processing is done on the chip and it outputs a current based on whether or not it detects motion.

int pin = 2;

void setup(){
  pinMode(pin, INPUT);

  pinMode(13, OUTPUT);
}

void loop(){
  int pirVal = digitalRead(pin);

  if (pirVal == 0) {
    digitalWrite(13, HIGH);
  } else {
    digitalWrite(13, LOW);
  }

  delay(100);
}