Usage
Clicking the 
button or inserting the Gliffy macro (described below) will create a new diagram. Each diagram has two links below it: 'Full Size' and 'Edit Diagram'. Clicking 'Edit Diagram' will bring up the Gliffy web-based editor where you can create and modify your diagram. When you're done, clieck 'Back to Confluence'; your diagram will be saved and you'll be sent back to the original Confluence page.
Diagrams that you create will be stored as an attachment to the original Confluence page. Each time you save your diagram in the Gliffy Editor, a new version of your diagram will be created and attached to your Confluence page. The diagram will retain the same name, so you'll have a complete version history of all changes. However, do not edit the attachment by hand or you will destroy the diagram and have to revert to a previous version.
The Gliffy editor also lets you export your diagram as SVG, PNG or JPG if you need to use it outside of Confluence.
Gliffy Plugin Video
Click Picture
Gliffy Macro
The Gliffy Macro is automatically created and inserted into your page when you click 'Add Diagram.' However, you can also insert it manually to reference a diagram attached to another page:
{gliffy:name=Test Diagram|space=~jnolen|page=test|align=left|size=L}
Terminology used in the macro:
Name The name of the diagram (and of the attachment)
Space The space in which the diagram is attached
Page The page to which your diagram is attached.
Align left, right, center
Size *T*humbnail, *S*mall, *M*edium, *L*arge