When you first learn about photography, you’re taught about the exposure triangle – shutter speed, aperture, and the ISO sensitivity of the film. Back in the day, you’d choose your film sensitivity to suit the light levels under which you’d expect to shoot. You then play aperture against shutter speed to balance the exposure of the image. You might then opt for smaller apertures (bigger f number) for longer depth of field means you need a longer shutter speed. Choosing a short shutter speed to freeze action would conversely mean you needed a bigger aperture to capture sufficient light.
In digital photography, the concept of ISO has changed. With a digital camera, you can have a different ISO for every frame you shoot if you like and so you have more options about balancing aperture and shutter speed for artistic purposes. However, it is important to note that while a different ISO value really meant a different light sensitivity of the film, you cannot change the sensitivity of a digital camera’s sensor. So, changing ISO is not changing anything physically as it would be with film.
Rather, changing ISO in a digital camera affects how the signal from the sensor is processed by the camera’s software. When you increase the ISO in a digital camera, the sensor’s raw signal is amplified. This means the image becomes brighter. The change in ISO is more akin to turning up the volume control on your stereo or headphones. And, as with a stereo turned up too loud you get distortion. For a digital image that distortion is, ironically, known as noise (random specks of colour in the image). Noise is more noticeable at higher ISO settings and in the darker areas of an image, which is why many photographers try to keep their ISO as low as possible.
The high-ISO noise effect is worse the smaller the sensor, so full-frame digital cameras tend not to be as “noisy” as cropped sensors and cameras with even smaller sensors, such as superzooms.
Of course, there are tools such as DxO’s PureRaw, Lightroom, Topaz Denoise, etc that allow you to remove the noise to some degree without distorting the image. Some work better than others, for me, PureRaw 4 seems to give me the best results.