When several items are in close proximity to each other, they become one visual unit, a cohesive group. The proximity implies a relationship. Similarly, separating items not related as an organizational strategy also provides clues in terms of content, in terms of the relationships, and in terms of the hierarchy of elements on the page.
Alignment creates visual connections. Similarly aligned elements can be grouped, and grouping can be implied by either aligning these elements to the same invisible line or by conforming these elements to the same alignment based shape. Visual relationships are created through alignment.
Repetition can be used to create rhythm through consistency, consistency can create coherence. Variation can then be used to draw attention. Organized variation can be used to create hierarchy and to influence rhythm.
Contrast is difference to varying degrees. Contrast works along a scale of difference to determine flow, to create a general speed of read. Contrast works along a scale of difference to determine the level of attention to detail your reader will give your design.