Its very simple. PageID and TabID are the same thing and refer to the unique primary key for each record in the Tabs table of the database which identifies the Page.
ModuleID is again the primary key of the record for that module.
You will see a table called TabModules that has the many to many relationship of Tabs(Pages) to maodules so that the same module can be displayed on multiple pages.
Philip Beadle (MVP, MCAD,
DotNetNuke Core Team)