Whenever we attempt to make sense of information visually, we first observe similarities and differences in what we are seeing. These relationships allow us to not only distinguish objects but to give them meaning. For example, a difference in color implies two distinct objects (or different parts of the same object), a difference in scale suggests one object is further from us than the other, a difference in texture (one is more blurry) enforces this idea, and so on. Once we have an understanding of the relationships between elements, we can piece together the whole story and understand what we are seeing.

Boxes and Arrows has an excellent article on Visible Narratives: Understanding Visual Organization