PIR "Passive Infrared"sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. 

How to Use PIR Sensor

The PIR sensor have 3 pins:

• Power:       3-5 VDC
• Output:      High  (when motion detected)

• Ground:    connected to ground

The output pin will go "high" to 3.3V and light up the LED when motion detected.

Specifications:

• Input Voltage:                 DC 3-5V
• Output signal:                 3.3V (motion detected-output high)
• Detection Angle:             110 degree
• Detection Angle:             max 7 m

How Does The PIR Sensor Work

The PIR sensor itself has two slots each one of them is made of a special material that is sensitive to infrared. When a warm body like a human or animal passes by, it first intercepts one half of the PIR sensor.  This causes a positive differential change between the two halves.  These change pulses are what is detected.

How to use PIR Sensor with Arduino, please click to download