+++ Overview
Doctrine is a tool for object-relational mapping in PHP. It sits on top of PDO and is itself divided into two main layers, The DBAL and the ORM. See the picture below.

[http://code-factory.org/doctrine-manual-images/doctrine-layers.jpg]

The DBAL layer completes and extends the basic database abstraction/independance that is already provided by PDO. The DBAL library can be used standalone, if all you want is a powerful database abstraction layer on top of PDO. [Link to DBAL download here].
The ORM layer depends on the DBAL and therefore, when you load the ORM package the DBAL is already included.

+++ The terminology used in the manual
**Entity class** : A class whose instances are persisted into a relational database (classes that inherit directly or indirectly from Doctrine::ORM::Entity).
**Entity** : An object that is an instance of an Entity class.
[More key words and short explanations]