Well in simple terms it is light we can't see with the naked eye. But it is there. The Infrared Spectrum is a look into an invisible world if you will. While the human eye can see wavelengths from about 400nm-700nm (purple to red) see the graph to the left. The infrared spectrum is beyond the 700nm wavelength we can see. The infrared light I capture is different than thermal IR, that looks farther into the infrared spectrum.
Every camera can capture Infrared light, but they have a filter in front of the sensor that blocks the light. It produces some very distinct effects that may have a tendency for people to either love or hate them. Mostly I believe because it allows us to see the world differently and not how we (our eyes) train our brain to see the world. Often in IR, we get a very surreal color landscape or dreamy black and white. IR allows me to push my creativity in a way not often seen in the area. I love the feeling of processing the image and getting it to come to life.