Balance refers to the weight of objects, and their placement in relation to each other.
Proportion is the size of objects in relation to each other, or within a larger whole. This could be natural (e.g. a nose which fits onto a face the way you would expect it), exaggerated (e.g. a nose that is vastly over
Emphasis is an extension of these first two principles: it is when contrast, placement, size, color or other features are used to highlight one object, area, or other elements of the artwork
Variety is a sense of difference between elements of an artwork – the opposite of unity, or harmony. Variety adds a sense of chaos to a work, and this is often used to highlight certain powerful emotions.
In follow on from variety, harmony is the use of related elements. This might be similar colors, shapes, sizes of objects, etc.
This indicates the direction your eye takes as you view the work – in what order does your eye travel? If emphasis is used, this often means you start with this element first and travel away from it.
Not to be confused with harmony, unity is the overall cohesion of the work. You might achieve this through any kind of grouping of objects. Any kind of similarity will help to strengthen the sense of unity you feel when looking at a series of objects.