In a nutshell, ISO stands for your camera sensor's sensitivity to light. Back in the old days when people shot with film, different film types had different ISO sensitivities.
Camera ISO's range from 100-40,000+
If you've ever noticed when you take pictures with your cell phone, pictures in low light settings are super grainy. That's your camera's sensor causing all that grain.
Each camera's ISO max is different. This is where the more expensive camera's take the lead.
The higher your ISO setting is, the lower the light you have the ability to take photographs in.
So, Mrs. B, why don't I just bump my ISO to full max to take low light photographs?
Well, kind of...
Every camera has a "sweet spot" as to how high you can turn up your ISO without losing quality in your image.
The higher the ISO, the "grainier" and "noisier" your image will be. The image on the right is an example. Compare the image shot at 125 ISO vs. 3200+ ISO. That sand-like or "dusty" look is what we call "noise." It is, in most cases, not an ideal effect you want on your images. However, once you decide what your aperture and shutter speed HAVE to be, the ISO is the last part of the exposure triangle you want to adjust.
Little to no grain
The more expensive digital cameras are able to go up to 3200 ISO and up without losing image integrity.
The more basic digital cameras usually cap out in quality around 800-1200 ISO.
ALL THE GRAIN.
Wedding photographers are a good example for photographers that depend on high ISO's to get the shots they need. They're frequently shooting in low-light situations, as well as indoors. They are constantly shooting at ISO's 1200+.
The image below was shot of a theatre program. I had to shoot at a high ISO of 3200 because I could not risk motion blur with a slower shutter speed. My aperture was also as wide open as I could allow without risking depth of field mis-focusing.
You might be wondering, "the images look fine zoomed out, who's going to be zooming in that close to tell if it's grainy or not?"
Great question! When you're wanting to print images or have them big in size for display, that's when all the little secrets start to show. Especially that good ole' missed focus point we discussed in the APERTURE section.
How high can you set your ISO before losing all quality?