ISO

 The images above show a difference in ISO when they were taken. one was in a lit room and the other was in a room with very low light. The image at the top was taken at 1/50 shutter speed but at 200 ISO while the one below was taken on the same shutter speed but at 1600 ISO. The increased ISO creates a noise as show in the bottom photo. This contrasts with the top picture which contains almost no noise to the naked eye. Different cameras handle noise differently due to there sensors, both of the photos i took there were taken on a Sony A77 which handles noise pretty well. 

What is ISO?

ISO is the ability to artificially raise the light in a photograph without the need to change any other settings and allows the user to brighten areas in a shot which might be to dark. ISO when put to high will create noise making the shots you take worse however if mixing ISO with shutter speed settings you can create more light in the sensor without needing to have super high ISO which will mean you won't get the noise.

Camera ISO