How Database Formats Files for Disk
I think it is opportune to start pointing out that, in a relational database —as per Dr. E.F. Codd— information is represented by means of one and only one structure, the relation, which is usually portrayed as a table. 

The relational model deals exclusively with aspects of the logical level of abstraction, so a table (an abstraction) is part of the logical level of a database (it is susceptible of undergoing logical operations, e.g., projections, joins, restrictions, etc.), so it is completely independent from the physical storage utlized by the database management system (DBMS for brevity) of use.

That being so, since the relational model has to do solely with logical-level aspects, it offers physical data independence, which sets DBMS (e.g., SQL platforms like IBM DB2, Microsoft SQL Server, PostgreSQL, etc. —that although quite powerful, strictly speaking, are semi-relational, i.e., they do not comply fully with the stipulations of the relational framework—) free to (a) store at the physical level of abstraction in any way their developers/vendors deem convenient (b) the data represented via logical-level tables. 

It is important to note that the physical-level storage can in turn be divided in distinct sub-levels of abstraction.

Therefore, the physical arrangements depend on the particular DBMS under consideration, hence they can vary from platform to platform.

Examples of physical structures utilized by different SQL platforms are detailed 


in the Storage Page Layout content from the PostgreSQL official documentation; and
in the post entitled Inside the Storage Engine: Anatomy of a record, regarding Microsoft SQL Server, by Paul Randal; and
in SQLite's Database File Format page.


In case you are interested, you may find of help this answer to a question about some of the different levels of abstraction of a computerized information system.