Similarity occurs when objects look similar to one another. The viewer will perceive them as a group or pattern. These similarities can be found in similar shapes, colors, sizes, and textures. This principle is probably the easiest to understand but also the broadest of the Gestalt principles.
When objects are similar, another object can be emphasized by making it different than similar ones. Then, you are automatically compelled to see the elements which are emphasized. This is known as the Anomaly theory and it is an additional feature of the similarity principle.