.twbx:
The .twbx file provides all of the information needed to create the visualization, as well as the data source. This is known as a packed workbook, and it compresses the entire bundle of files.
.twb:
The .twb file has instructions for interacting with the data source. When creating a visualization, Tableau examines the data source before creating the graphic using an extract. It can’t be shared on its own because it simply provides instructions, and the data source must be added separately.