Cameras capture moments through light—ISO determines just how light sensitive a camera is. In film, the ISO isn't a camera setting, but rather the film speed. But what happens when you swap out film for a sensor? What is ISO on a digital camera?
The ISO setting on a digital camera (or camcorder) indicates how sensitive the sensor is to light. A high ISO, of say 3200, means the camera is highly sensitive to light. A candlelight image taken at ISO 3200 will appear brighter than the same image taken at ISO 200. Higher ISOs allow the camera to gather more light, resulting in a brighter image overall.
Why not use high ISOs all the time? The trade-off of a high ISO is greater noise, or a grainy texture to the image. A photo shot at ISO 3200 will be brighter than one shot at 200, but will also have more noise.
To complicate things a bit further, not all digital cameras perform equally at high ISOs. ISO stands for International Organizational Standard, and while ISO 100 on one camera is the same setting as ISO 100 on another, two different cameras can have very different results, particularly at high ISOs. Generally, cameras with larger sensors will have less noise than a camera with a smaller sensor at the same ISO settings, but size isn't the only factor. Sensors that are back-lit, or that place the wiring behind the sensor instead of in front, perform better at higher ISOs than similarly sized sensors with a typical construction.
And while most people perceive more megapixels as a good thing, that's not always a case, particularly when it comes to ISO and low light photography. A 16 megapixel camera with the average 1/2.3” sized sensor will have more pixels crammed into a smaller area than a 16 megapixel camera with the APS-C sized sensors used in DSLRs. Which one will have less noise? The larger sensor, because of the lower pixel density. Put simply, the larger sensor has larger sites for collecting the light, called photodiodes, so low light images will have less noise. The best cameras for high ISOs (or for low light scenarios) will have large, backlit sensors that aren't overloaded by too many megapixels.
ISO is just one aspect of digital photography that affects an image's exposure (or amount of light). Shutter speed also affects exposure; the longer the shutter is open, the more light that can reach the sensor. Aperture alters the amount of light reaching the sensor by changing the size of the opening. All three together ultimately determine the amount of light in an image, and all three are adjusted using manual modes on advanced, mirrorless and DSLR cameras.
The best ISO settings are dependent on the situation. Generally, if there's plenty of light, a low ISO is best, though higher ISOs can be used even with efficient lighting to purposely add grain, which tends to make an image look aged. Most cameras should avoid ISOs of 3200 or higher, but some good low light performers can handle even ISO 6400 fairly well.
Light is essential to photography—the word photography literately means writing with light. So what is ISO? ISO determines how sensitive the camera's sensor is to light, though other factors like the sensor size and type as well as aperture and shutter speed all impact how a camera records light.