A Screen is a collection of screen sections and other widgets built on a data structure definition.
This means that a form can display different entities and specific rows of these entities. Screens make also use of the relationships between the entities, which allow seamless master-detail navigation scenarios. Creating multi-level master-detail forms is very easy.
Screens are mostly assembled from screen sections (screen section instance is a link to a Screen Section Definition so you can reuse screen sections in multiple screens). To make the user interface more intuitive and readable for the user, additional widgets like Tab Control or Split Panel can be used.
Extensibility
If you are using a package which has a screen defined and you want for example, to add a new tab to the screen with your own customized section or you want to provide your screen section instead of the provided one you can do so by using Alternatives.
You can create a screen alternative by right-clicking on your screen element and selecting New > Alternative. A new version of the original screen will be created in your package. This version will be used instead of the original version. You can make as many edits to it as you want. If the original screen is updated (new widgets added) your version will stay intact (you will need to add those widgets manually for the screen alternative you created).