The Relational Database Model
A database can be understood as a collection of related files. How those files are related depends on the model used. Early models included the hierarchical model (where files are related in a parent/child manner, with each child file having at most one parent file), and the network model (where files are related as owners and members, similar to the network model except that each member file can have more than one owner).
The relational database model was a huge step forward, as it allowed files to be related by means of a common field. In order to relate any two files, they simply need to have a common field, which makes the model extremely flexible.
In 1970, when E.F. Codd developed the model, it was thought to be hopelessly impractical, as the machines of the time could not cope with the overhead necessary to maintain the model. Of course, hardware since then has come on in huge strides, so that today even the most basic of PC's can run sophisticated relational database management systems.
http://www.databasejournal.com/sqletc/article.php/1469521/Introduction-to-Relational-Databases.htm
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