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The Data module lets you use database tables that are foreign to Drupal.
You can 'adopt' a table in your database that ordinarily Drupal would not be aware of. This might be external data you have imported, or a table that another application has created. You can modify this table's schema in Data's UI, and you can also create custom tables in the Data UI.
A table that Data is aware of can be shown in Views. It can also be declared as the base table for a custom Entity type. This allows Field API fields to be added, and relationships to be created to other entities with Entity Reference fields.