Containers hold sets of forms and/or elements; depending on the type of container, users can often move these around their screen to accommodate for better visibility.
Dialog boxes have two fundamental types. Ultimately, the decision on what type of dialog box it should be is dependent on the type of work the agent will - or won't - need to be able to do while accessing the dialog:
Modal dialog boxes, such as the left-most example, require users to complete and close the window before continuing with the owner window. These dialog boxes are best used for critical or infrequent, one-off tasks that require completion before continuing.
Modaless dialog boxes, such as the example on the right, allow users to switch between the dialog box and the owner window as desired. These dialog boxes are best used for frequent, repetitive, on-going tasks.