Toolbar

The toolbar at the top of the document window provides easy access to controls of your choosing. Most importantly, the toolbar is home to the tool palette you use to create diagrams. By default, it also contains the controls we think you'll find most useful. Select Customize Toolbar... from the View menu for a sheet containing controls you can drag to and from the toolbar, and to choose the size and style of controls you would like to see. You can show and hide the toolbar using the white button on the right side of the title bar.
Most toolbar controls are just convenient ways to access menu commands or inspector controls. The main exception is the drawing tools palette, which exists primarily in the toolbar (though you can detach it from the toolbar via the Drawing Tools preferences).
The Utilities button, which shows and hides the utility drawer, can be Option-clicked to toggle on which side of the document window it appears.
AppleScripts in your application support folder are also available as toolbar buttons when you customize the toolbar. Just click a script's toolbar button to run it.