This first chapter has given you the means to understanding the basics of creating diagrams in OmniGraffle and then exporting your diagram for other users.
You have also learned to:
Create various shapes like oblongs and diamonds
Connect two or more shapes with lines
Add arrow-heads to lines
Add text both to shapes and labels to lines
Create basic styles for shapes
Copy partial or complete styles from one shape to another
In the next chapter, you will learn everything there is about stencils. A stencil is a collection of ready-made shapes. OmniGraffle has very good support for stencils and using these will save you a heap of time and frustration.