Contrast-
Contrast is one of the more important concepts in my opinion. It is also my favorite. If contrast is achieved it can make a piece much stronger. It makes someone see instantly what should be seen first, second, etc. What do I mean by contrast? A lot of things...
Text

When it comes to text, contrast is important to grasp the reader's attention. It forms hierchy of what is the most important points and what goes from there. From this example you can see that the one on the left is pretty generic and dull. Creating high differences in point size (go all out, don't be a wimp), bolding text, or starting the first letter larger will make the entire presentation of text more interesting to read.
Color

To create high contrast in color while maintaining harmony it's best to use colors that create a lot of opposition to each other. Such as using dull with bright colors, dark with light, or as this example shows on the right - complementary colors (those opposite on the color wheel). The design on the left shows that similar colors look nice but are more fit for a formal or calming design. Contrasting colors can be used to create depth and once again create hierchy of importance to a piece.
Shapes


To create contrast in shapes use differences like sharp with rounded shapes. Use them at different angles. Use different sizes. As you can see from the 1st example, creating an object with a different size will grab the viewer's attention a lot more than the monotony of the design on the left of number 1. In the 2nd example you see that the one on the left has all shapes of about the same size and proximity to one another, *yawn*. In the one on the right the design comes alive with music. It dances and shows energy. Shapes of different sizes going at different angles creates an interesting mix.
- I hope this lesson has been helpful. These are the bare basics of graphic design. The CRAP principle is something everyone should memorize who is interested in this field. Next lesson will be posted in about a week. If I have made any mistakes please contact me. Comments and opinions are welcome in the reply to this post.