PIR stands for Pyroelectric ("Passive") InfraRed sensor
For a brief explanation on what PIR is and how it works, follow LadyAda's tutorial here.
For this project, you will require
A PIR sensor ( Dome preferred )
An Arduino board. (I used the Arduino Mega 2560)
USB cable to program the Arduino
The arduino IDE correctly installed on your computer
I got all my parts from Element14 here
Connect VCC to 5v on the Arduino.
Connect Ground to GND on the Arduino.
Connect the Digital OUT of the PIR sensors to Digital Pin 2 on the Arduino
The output on my serial monitor was: