As defined as the sensitivity of your camera’s sensor, or film, to light.
Quickly put, ISO allows me to increase my shutter speed, or close my aperture to get a better exposure.
This is achieved by setting your ISO via the dials on the top of your camera, the ISO wheel on most DSLRs, or by using a different film in your camera.
For example, It may be a little dark at a party where I want to shoot some photography of what’s happening, but my shutter speed is too slow and is making my images blurry and I cannot open up my lens aperture any more to let more light in – so, I’d increase the sensor’s sensitivity to light – which will then allow me to shoot at a shutter speed which will work better for my images.
This comes with a side effect known as grain, or noise. Because the sensor (or film) is more sensitive, more information is scattered across the sensor and the camera does its best to clean that up as best it can, but, it leaves noise. The lower the ISO, the less noise in your shot (generally speaking).
There are circumstances where you want grain. Maybe you shoot street photography and want to add it to emulate an older film camera, or perhaps you just need more light and are happy to try to clean it up in post processing. It could just be that you wanted a picture of something so bad you didn’t care how grainy it was – for example, a picture of you and a ghost. Proof!
ISO ranges from 50 to about 51200 (or higher for newer cameras) – the lowest being the cleanest in terms of noise. Most cameras these days can handle 3200 ISO no problem – but above that, things start to get worse for the image.
In the film days, you can’t change the ISO until you changed the roll of film. You bought the film which is rated for a certain ISO sensitivity. You could PUSH the ISO though by a stop or two and that would get you some added noise, but you have to remember what you set before you develop the negatives!
Of course, artistically, it’s up to you how you use grain and noise – I am just providing an example. But sometimes, you have to accept it if you want to achieve a certain shutter speed, remain at a certain aperture or just get the shot.
Many cameras now in the digital age use AUTO ISO – this lets the camera choose the ISO setting. You can provide a minimum and a maximum allowed ISO and the camera will do its best to stay within those limits.
In the below example of grain, you can see that the lower the ISO, the less noise in the image there is. As the ISO is increased, so does the noise.
For night photography, you may want to have a lower ISO setting because you know your shutter speed will be pretty long.
Shooting at a concert – it’s probably going to be fairly dark, so you’ll want a higher ISO setting to be more sensitive to the light that’s there.
Of course, photography is purely subjective to what the photographer wants to show the viewer. The only technical aspects are just choices. No right or wrong.