Infrared detectors

[Fredinappolis]

I made an infrared detector circuit with the little infrared detector and emitter from radioshack. My problem is that I don't understand how the detector works. Can it detect any infrared radiation, or is it just for the emitter that comes with it? I just need to understand the theory behind it. Does anyone have any good websites that describe how this works? Thanks.

[xxconfusedxx]

as in something similar to a PIR on a burgler alarm?