Principles of Design
Unity
Unity is when all of the design elements come together to form one unified image. Unity can be acheived by:
Using simular or complementary color
Using simular shapes
Proximinity, bringing the objects closer together.
Overlapping objects
Another term for the same idea is Harmony. If the various elements are not harmonious, if they appear separate and/or unrelated the design lacks unity.
Although you can still have unity when using a group of different shapes, unity would depend upon the position of the shapes in relationship to one another. Using colors that are not harmonious can work against a unified design.
Balance
Balance requires some equal distribution of visual weight.
Symmetrical Balance
The objects are repeated in the same position on either side of the design. One side is a mirror image of the other.
Sometimes while attaining perfect symmetrical balance you can loose unity and visual interest.
Asymmetrical Balance
An asymmetrical design is normally more interesting.
Once objects on one side of the image are heavier it is important to create ways to move the viewer’s eye around the design.
Asymmetrical balance can be achieve by the placement of color, texture, lines.
Emphasis / Focal Point
Emphasis is also called Focal Point or Dominance.
The viewer is shown the most important object within the design immediately. In our case it will probably be the client’s name, logo and/or product.
Often the design of your logo will be enough to draw the viewer’s attention.
Our eye is drawn first and foremost to images of faces, and you need to be careful that the pull towards the face is not so great that the viewer’s eye does not take in the rest of the design.
The eye will also follow the direction in which a face is looking.
You do not want the user's eye to be taken outside of the design, therefore you do not want a face or an object to point outside of your design
Red has a greater pull on the eye than any other color. 
In most designs you will need to use red sparingly
Rhythm
Rhythm is a basic design principle based on repetition. It involves a repetition of elements that are the same or slightly modified.
Alternating Rhythm
Alternating rhythm is found in these blue and white lines
Progressive Rhythm
The shapes change but in a fairly regular manner.
Progressive Rhythm often has an organic feel..
Rhythm is extremely important in interactive projects. The repetition of placement and alignment from interface to interface for the logo, menu and content needs to remain consistent, abrupt changes in the placement of design elements from one interface to another are often quite jarring to the user.
Scale / Proportion
Proportion is the relationship in size between one object and another.
It can be used to establish distance, the smaller object in the example appears to recede.
You can exaggerated or reduce the size of an object to make a point.
Change of an expected scale can attract the users attention.
