SparkFun Simple Sketches - Magnetic Reed Switch
Reed Switch: https://www.sparkfun.com/products/8642
This is a small device called a reed switch. When the device is exposed to a magnetic field, the two ferrous materials inside the switch pull together and the switch closes. When the magnetic field is removed, the reeds separate and the switch opens. This makes for a great non-contact switch. This switch can carry up to 1.2A.