Jaggies & noise
Enlarging a digital image or examining it from close by will reveal visible steps in diagonal lines in an image. Since pixels are square in layout every diagonal line has to be built up by square blocks of color stepped on top of one another. These “steps” are called jaggies and are usually visible on lines at slight angles against a contrasting background. The effect is more prominent in low resolution images since less pixels make up the image and consequently the “steps” to form diagonal lines will be larger.
Visibility of jaggies can be reduced by aliasing lines against their background so they look more like a smoother line. When applying unsharp mask however, they may reappear since sharpening is often based on selective contrast adjustment which removes aliasing.
Other forms of noise can be compared to grain or imperfections in conventional film. They are caused by electronic errors or intererference of the camera's CCD or CMOS sensor. Certain color channels are more affected by noise than others because digicam's sensors are more sensitive to certain primary colors than others.
Visibility of noise is often affected by ISO setting or temperature. Higher ISO speeds or temperatures amplify noise while long exposures can also introduce noise in an image. Since JPEG compression reacts badly to noisy images this can amplify noise even more.