PIR sensors are used to detect motion from pets/humanoids from about 5 meters away (possibly works on zombies, not guaranteed). This sensor is much smaller than most PIR modules, which makes it great for unobtrusive projects. It's also fully-contained - only needs power, ground and a resistor to set the delay time - and breadboard or perfboard friendly.
Runs on 3.3V power - so make sure if you're interfacing with an Arduino or other 5V chip, you power it from the 3V regulator. Digital signal output is 3.3V high/low and you can change the length of time the signal pin stays high by connecting a resistor divider to the second pin, from 2 seconds (0V) up to one hour (3.3V) - see the datasheet 'on time' table for recommended resistor values.
To use, see datasheet page 7 pin-out diagram: