ISO represents the sensitivity of image sensor to the light.

 

ISO:

Higher ISO = More sensitive to light.

image

 

Noise:

Higher ISO = More Noise

The main disadvantage of using higher ISO is the noise generated. Both images below are cropped images.

image
Canon Rebel XSi

image
Canon PowerShot SD750 (Consumer Digital Camera)

 

Conclusion:

Lower ISO provides better image quality.

Places without sufficient light (indoor or night) may require higher ISO to reduce camera shake.