Dialogs are a specialized version of {@link com.smartgwt.client.widgets.Window} used for small windows that contain justa text message or a text mesage with some standard buttons.
Many typical modal dialogs such as alerts and confirmations are built into the system with convenience APIs - see say, warn and askForValue.
Dialogs can be modal or non-modal according to {@link com.smartgwt.client.widgets.Window#getIsModal isModal}.
NOTE: If you are building a dialog that will involve more than just buttons and a message, consider starting from the {@link com.smartgwt.client.widgets.Window} class instead, where arbitrary components can be added to the body area via {@link com.smartgwt.client.widgets.Window#addItem Window.addItem}.