ISO

What is ISO?

ISO is the camera's sensitivity to the available light.

How does ISO work technically (in the camera) and visually (in the photo)?

In the camera, altering the ISO changes the sensitivity of light. When the ISO decreases, the camera is less sensitive to light, so more light is needed. When the ISO increases, the camera is more sensitive to light, and less light is needed. A lower ISO has a better quality, while a high ISO produces a lower quality image. Visually, a photo with a lower ISO is high quality with a less apparent grain. A photo with a higher ISO is low quality, and it shows grain. 

What happens to the exposure triangle when you change your ISO?

When the ISO is changes, the camera's sensitivity to light is altered. At a low ISO, the camera needs more light. To balance the light meter, the shutter speed should lower to let more light in. However, at a high ISO, less light is needed. In order to compensate for the greater sensitivity to light, the shutter speed should increase to let in less light.