ISO is the measurement of the camera's sensor/film sensitivity to light. It is one of the 3 ways to create a well exposed image (The other two being Aperture and Shutter Speed). Typically, a higher ISO for a correctly exposed image will result in the image being more noisy/grainy.
If ambient light isn't enough and I don't have a tripod with me for long exposure, which one should I prefer to increase the brightness of my photo? I know high ISO comes with noise. Is there any ...