First of all, calling it just infrared photography would actually be false, the proper term for IR would be NEAR infrared. However, its a lot simpler to just called it IR and drop the NEAR.
Let's look at what infrared light actually is.
In infrared photography, we take pictures of near infrared light, ranging from visible light of 590nm, all the way up to 980nm which is past the visible spectrum. Anything above 1000nm is used specifically for thermal imaging and not related to infrared photography.
Regular cameras usually only take visible light. As the picture above states, it is what is visible to the human eye, between 400nm to 700nm. This is because most digital cameras have something called an infrared cut-off filter placed on top of the camera sensor, as camera sensors are actually sensitive to infrared light. Meaning that without that IR cut-off filter your camera can take infrared pictures, as it let's in anything above 700nm. However, this would not be great for regular photography as your camera is now full-spectrum, letting in both visible light and infrared light (refer to full spectrum picture below).
Above is an example of an IR cut-off filter that's on top of the sensor.
For infrared photography, we want to capture only infrared light, and not regular light. To do this we have something called infrared filters. I'm sure you are aware about filters that you use to mount onto the front of your camera lenses. For IR this is the case as well, we take a filter and put it onto the front of our lens, the alternative, which some individuals prefer is to have an infrared filter placed directly into their camera on top of their camera sensor.
Above are some infrared filters.
By placing these filters onto the camera lens, we are now blocking out all light below a certain wavelength. For example, if we place a 720nm filter onto our lens, we are blocking out all light below 720nm, thereby blocking out visible light as according to the infrared spectrum picture above.
So, basically infrared photography is capturing none-visible light spectrum above a certain wavelength, typically starting from 550nm onwards. Different filters give different effects, you can find a comparison of the different wavelengths of IR filters here.
Visible light (Only light between 400nm to 700nm)
Full Spectrum (Visibile light & IR light)
590 nm filter (only IR light above 590 nm)
As you can see, the third picture above shows the near infrared spectrum at 590nm. This is what infrared photography is known for. You get a surreal perspective of the world.
For a basic guide on how to get into infrared photography, you can check out this simple guide here.
For a more in depth look into how infrared light works, you can continue reading this advanced guide here.