Balance creates the perception that there is equal distribution. This does not always imply that there is symmetry.
Contrast focuses on making items stand out by emphasizing differences in size, color, direction, and other characteristics.
Scale identifies a range of sizes; it creates interest and depth by demonstrating how each item relates to each other based on size.
Dominance focuses on having one element as the focal point and others being subordinate. This is often done through scaling and contrasting based on size, color, position, shape, etc.
Similarity refers to creating continuity throughout a design without direct duplication. Similarity is used to make pieces work together over an interface and help users learn the interface quicker.