In digital cameras, the ISO sensitivity is an indicator to show how much the light coming from the lens is amplified in the camera.
The amount of light coming into the camera is determined by the aperture and shutter speed. From this amount, the light is amplified to create a well-exposed image. The ISO sensitivity represents the level of this amplification numerically. For example, ISO200 is twice as sensitive as ISO100. This means that adjusting the setting to ISO200 lets you shoot with the same brightness as ISO100 even in half the amount of light.
Actually, in most modes, the ISO sensitivity is automatically determined by the camera according to shooting conditions. However, in the P/A/S/M-modes, you can also set it manually depending on your needs.
As the sensitivity gets higher, you can use faster shutter speeds even in low-light situations to reduce blurs caused by camera shake under low light or subject movement in sports shooting. However, because the light is amplified electrically, shooting with the high ISO sensitivity tends to result in more noise (grain) or loss of sharpness on the photograph.
 ISO: 3200 ISO: 800
The above night view photographs were shot by holding the camera by hand, with ISO3200 for , and ISO800 for .
By adjusting the setting to a high sensitivity, an image blur was prevented, but the building looks grainy in the photograph on the left.