ISO

In traditional film photography, ISO was the indication of how sensitive a film was to light, it was measured in numbers and the lower the number the lower the sensitivity of the film and the finer the grain in the shots you are taking.

In digital photography, ISO measures the sensitivity of the image sensor. The same principles apply as in film photography, the lower the number the less sensitive your camera is to light and the finer the grain. 

Here are two examples of ISO, the image on the left was taken with 100 ISO and the image on the right with 3200 ISO. You can see the difference easily.