What is a Tableau workbook?

A workbook is the file that holds your work — worksheets, dashboards, stories, data connections, calculated fields, parameters and formatting.

It saves as .twb, which is XML describing all of that but containing no data, or .twbx, which packages the same thing together with a copy of the data.

Structurally it is a container: sheets hold individual views, dashboards combine sheets, and stories sequence sheets or dashboards into a narrative.

It is also the unit of publishing — you publish a workbook to Server, and permissions and refresh schedules attach at that level.