Architectural pattern for separating application logic from storage.
Objects and relational databases have different mechanisms for structuring data.
The Data Mapper is a layer of software that separates the in-memory objects from the database. Its responsibility is to transfer data between the two and also to isolate them from each other. With Data Mapper the in-memory objects needn't know even that there's a database present; they need no SQL interface code, and certainly no knowledge of the database schema.
Pattern definition: http://martinfowler.com/eaaCatalog/dataMapper.html
