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A visual comparison is to compare two or more things by eye. This might be done by placing them side by side; by overlaying them; by alternating an image or by presenting each image to a separate eye.
Such comparisons are the first stage in a child's development of an understanding of geometry and measurement, before they move to an understanding of measuring devices such as a ruler.
People with sufficient control over the parallax of their eyeballs can hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware and differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artefacts.
Visual comparison with a standard chart or reference is often used as a means of measuring complex phenomena such as the weather, sea states or the roughness of a river. A colour chart is used for this purpose in many contexts such as chemistry, cosmetics, medical testing and photography.