A Page-Widget is simply the link between a widget and a page. Or in other terms, it represents the placement of a widget on a page. A widget can be a form, grid or summary type component, and a page-widget puts an instance of widget onto a form. Architected this way, a widget can be used several times by linking it to many pages.
Within a page-widget you can also define default behavior for the particular widget it links to.
- Page-widget-filter: For example if a widget shows the portal user a case form (a customized CRM case form if you will), then you may want to include lookups on the that form. The lookups must also be filtered to show the portal user a specific set of records. If might you want the "Responsible Contact" lookup field on the case form to only list contacts who are also employees of the portal user's parent account. You configure that as a page-widget-filter.
- Page-widget-variable: For example, you wish the case form shown to the portal user to have certain default fields set appropriately, you create a page-widget-variable to define those.