Noise is the digital equivalent of grain in film. It’s random electrical signals captured by the photosites on the camera sensor, and usually this background noise level is so low compared to the brightness of the captured picture itself that you just don’t notice it.
Signal to noise ratio
Signal to noise is the technical term for the noise level in a digital image. The 'signal' is the image itself and the 'noise' is the random speckling effect you see in high ISO images. The higher the signal to noise ratio, the less visible noise will be in the image.