The Server Side wrote:
The DAO layer offers a great enhancement in the process of software development. It effectively decouples the business layer from the persistence mechanism. For this reason it gives robustness to our application's architecture and flexibility when it comes to the unavoidable evolutions and branches of our work. In fact, changing the way data is persisted (e.g. JDBC - possibly between different vendors, XML, files) is simply a matter of writing new DAO implementations in a well isolated layer.

But with the introduction of an ORM, well, things change quite a lot. Imagine the following scenario: we've got an application developed with a well-isolated DAO layer that interacts with a database via JDBC. Now we want to switch to an ORM solution (e.g. Hibernate) by writing a new DAO implementation which makes calls to our ORM.