Responding to a broken laser beam? Firstly, I must apologise as I'm a complete noob but the Arduino kits / lego pieces are fantastic and I can't stop tinkering.  Unfortunatley my knowledge does not match my passion.

I picked up a laser from Amazon: FamilyMall(TM) KY-008 Laser Transmitter Module 650nm 5V Diode For Arduino AVR PIC.

I'm wondering what the middle pin is good for and if I can somehow manage to detect and respond to the beam being broken? 

Thanks for looking. :)
Without a data sheet or the device on hand not sure what the middle pin is used for.  On the topic of detecting a broken beam the item you purchased is purely an emitter, in order to detect the beam being broken there are two main ways, using a photodetector on the "opposite end" of the beam but this means having wires trailing the same path as the laser beam, alternatively you could mount a mirror where the beam lands and reflect it back to a photodetector mounted close to the laser diode.  You would them use the arduino to detect the state of the photodetector.