Balance: I'm guessing you know what balance is, but in terms of interior design, it's achieved when careful consideration is given to the placement of objects within a space. The reason for this is to give the sense of stability and a sense of equilibrium.
Rhythm and Repetition: Rhythm is created through repetition of line, form, colour or texture to create a visual link which the eye follows. It allows the eye to move from one part of a design to another part.
Proportion and Scale:They're both dependent on size and relationship between objects. Proportion does not depend on a known size. It's the relationship of one object to another, or one part of an object to its other parts or the whole. Scale is largely based on perception and compares an item or space to something of a known size. Human scale is the most common reference.
Harmony: This is the end result achieved when all the design elements and principles interrelate and work together to enhance the visual appeal of a space. Balance, proportion and rhythm in a space bring the elements of color, pattern, texture and shape into a functional relationship with the physical aspects of space, light and structure.
Emphasis: This refers to a focal point and supportive furnishings that create the centre of attention. It should be the accent to which your eye is attracted, and is often a view, fireplace or artwork. Without emphasis or variety, rooms are monotonous.