Medium can be broken down into 4 sub-categories.
Medium - ISO - Format - Resolution
The medium is what we record our images on. Traditionally it was film, but in today’s digital cameras it’s either a CCD or CMOS microchip known as a sensor. The photograph used to burn into film, but with digital it’s a little bit different. The sensor captures and records the light, it then converts that light into digital RGB values which make up the coloured pixels. Those pixels are then saved as a digital file. Either JPG or RAW.
The format is simply the size of the medium. Large format film, medium format digital etc.
The resolution is how much data we can fit in the image. This is basically how many pixels your image contains. 24 megapixels = 24 million pixels. Typically, the more megapixels, the larger you can print an image.
ISO is the sensitivity to light of our recording medium. It allows us to boost our level of light beyond what the standard settings can produce. The higher the ISO the brighter but more noisy your image will be (noise is grain). The lower the ISO the darker but more clearer your image will be. If you want a brighter image play with the aperture and shutter speed. The lower your ISO the larger you can print your images with minimal noise.
ISO allows us to shoot in different lighting conditions. If we were shooting with a standard ISO, say 100; and the light was very low, we’d be shooting with a slow shutter speed and our aperture might be at it's largest (maximum aperture). In this case we might find that we still can’t take the picture because the light is so low, so we would increase the ISO to gain more light.
Going from ISO 100 – 200 is doubling the exposure by 1 stop. You could say you’re digitally opening the aperture further by 1 f stop.