Gestalt theory

Similarity 

When objects looks similar to one another, viewers will often see the individual elements as part of a pattern or group. This effect can be used to create a single illustration, image or message from a series of separate elements.

Attention is drawn to the different element in this composition because it breaks the pattern of similarity

The similarity between different elements can be shape, colour, size, texture or value. The more commonality that individual elements have, the greater the sense of coherence, thanks to similarity.

A particular element can be emphasized when it's dissimilar, breaking the pattern of similarity. This effect is called an anomaly.

Continuation

In this example the eye is led through the first design element towards the star that aligns with the curve

Continuation is the principle through which the eye is drawn along a path, line or curve, preferring to see a single continuous figure than separate lines. This can be used to point towards another element in the composition, and is seen where a line is cut through one object, often in a curve, aligning perfectly with a secondary element.

Closure

The World Wildlife Fund logo uses the closure principle to describe a panda, even though the shape is not fully closed

Closure is a common design technique that uses the human eye's tendency to see closed shapes. Closure works where an object is incomplete or the interior space of an element is not fully closed, but the viewer perceives a complete shape by filling in the missing information. This technique is often associated with stenciled artwork, but is also closely associated with logo forms.

Proximity (aka grouping)

Proximity uses the close arrangement of elements to create a group association between those objects. If individual elements are also similar, they will tend to be perceived as a single whole, even though they are separate elements.

Symmetry and order

Put simply, this principle says that a composition should not provide a sense of disorder or imbalance, as otherwise the viewer will waste time trying to locate the missing element, or fix the problem, rather than focusing on the message or instruction.

You can achieve symmetry by providing a good balance or sense of symmetry in your design elements, such as the windmill illustration below. This provides the viewer with a feeling of harmony.