Data Mapping for Importing data from external database
During installation the CiviCRM are generated from xml files, and those will not always correspond 1-1 with the tables AFAIK.

You can find the xm definitions in the civicrm folder: civicrm/xml/schema.

Alternative is to use the API Explorer (Support/API Explorer in your CiviCRM menu) and do a getfields action for the entity? I would recommend using the API to do a migration anyway rather than moving into the tables directly.