The structure (columns) of each table is dependent on the fields contained in the object.
To get a list of objects available to your organization, refer to the Salesforce Get a List of Objects documentation. After you have a list of objects, check out the Entity Relationship Diagram (ERD) section of Salesforce documentation to see how entities relate to each other.
At this time, Salesforce does not currently expose the following objects in their API:
External objects - What is an External Object?