There's a lot more to design than what meets the eye. It's not just a random act of putting together images, words, and colors. Designers are guided by a set of universal principles and their sense of esthetic.
Contrast—the difference between design elements. Its main goal is to distinguish objects from one another and make them stand out, which is achieved through color, size, shape, or form.
Balance—the distribution of the visual weight of objects, colors, texture, and space in a design. It can be symmetrical with equal visual weight on both sides of a central line and asymmetrical with unequal visual weight on each side of the composition.
Emphasis—making certain parts of a design stand out or not stand out compared to other elements using contrast, balance, color, size, white space, etc.
Proportion—the relationship of sizes, shapes, and quantities between one part of a design and another part or to the whole composition.
Hierarchy—the ranking of design elements used to show the importance of each by manipulating colors, size, and perspective.
Repetition—the act of repeating the same or similar design elements like colors, fonts, lines, shapes, sizes, formats, and texture and making them come together as a cohesive whole.
Rhythm—the creation of a visual tempo in a composition. The repetition of elements such as lines, shapes, and colors suggests movement and provides a path for the viewer's eye. It's used to entice emotions of calm or excitement.
Pattern—the regular arrangement of repeated same elements (lines, shapes, colors) or setting a standard design for certain features.
White space—empty or "negative" space in a design whose primary purpose is to let the composition breathe or make some elements stand out.
Movement—the path a viewer's eye takes through the design, leading from one focal element to another. It's achieved through positioning, emphasis, etc.
Variety—the placement of different visual elements next to one another, preventing monotony through juxtaposition and contrast.
Unity—the concept of letting all other principles unify harmoniously within a composition, allowing each element to coexist, forming an aesthetically pleasing design.