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 from thermal IR, which 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.
These are before and after infrared images, the one that has the overall red appearance is before. After has the blue and purple hue. The really neat thing about IR is that no matter what the image looks raw from the camera, there is the ability to get creative, and once you tweak a few settings, the image shows me which way to go.