On this B&H video (made by Kelby Training) photographer Larry Becker explains the basic concepts behind ISO and cameras sensitivity and what you should know about them.
In the film days if you wanted to make your camera shoot in a brighter or darker situation one of the things that you could do (besides playing with your aperture or shutter speed) was changing your film. Back than films came with a number called ASA – the lower it was the less grain you used to see and lower ASA films typically used for shooting in sunlight while higher ASA films where used in lower light.
Modern day digital cameras don’t have ASA but they do have ISO which is very similar in some respects. Lower ISO will make out images with less noise for when you have more light and higher ISO’s will produce more noise but can help you shoot when there is less light.
Most current cameras start at ISO 100 (although pro cameras such as the D810 for example start at ISO 64 which gives them some advantage in image quality for daylight shooting). Currently many cameras reach their max ISO at 6400 with expended ISO going up to 25,600 (of course some pro cameras goes way higher and in some cases over ISO 100,000). However you need to understand that if camera X can reach ISO 6400 it doesn’t mean that it performs like another camera at the same ISO. Each camera has its own actual sensitivity performance (dependent upon the sensor, the processor etc.) and typically larger sensors will perform better than smaller ones (and newer cameras from the same manufacturer with the same size sensor will perform better than older ones from the same manufacturer with the same sensor size).