The guests will see whatever the presenter is currently sharing on their screen at the time.
If the presenter is using a Windows computer and joined the meeting using the GatherPlace software (GP4), then they can share one or more applications or their entire desktop.
If the presenter is using a Macintosh computer, or joined the meeting from a web browser on either a Windows or Linux computer, they must share their entire desktop.

The presenter also has the option to pause the presentation. The windows that the guests have been viewing the host's screen in will close while the presentation is paused and re-open when sharing is resumed.

For presenters using the GP4 software, there is a quick way to see what the guests are currently seeing by selecting the What Am I Sending menu item under the Help menu of GP4 which will open a small viewer window displaying a scaled image of what the guests are seeing. The Presenter must be sharing for the What Am I Sending menu item to be available in the Help menu.

Selecting the What Am I Sending menu item

The What Am I Sending dialog.