Principles of design

The two main principles of design are 

AESTHETICS and FUNCTION.

Aesthetics - The set of principles concerned with the qualities of appearance, visual appeal, good taste, and beauty and the rules that determine how beautiful or pleasing to the eye something is. Design elements within this principle include shape, form, colour, texture, finish, environment, point, line, plane, proportion, contrast, pattern, movement, balance, harmony, style, and rhythm.

Function - How a product, system, or environment works or performs for its intended use or user; how something carries out its purpose. Key elements include strength, durability, efficiency, safety, stability, reliability, ergonomic fit, construction (and its cost), optimisation, user-friendliness, and fitness for purpose.

On this page you will find a document that explains further principles related to those of aesthetics and function. It is important that you show and demonstrate understanding of these in your work. 

There is also an awesome resource from  http://teaching.ellenmueller.com that includes great explanations and examples of them with lots of visual examples.