Doctrine is the low level of configuration that is needed to start a project. Doctrine can generate object classes from an existing database, and the programmer can then specify relations and add custom functionality to the generated classes. There is no need to generate or maintain complex XML database schemas, as seen in many other frameworks.
Doctrine is the ability to optionally write database queries in an OO SQL dialect called DQL (Doctrine Query Language) inspired by Hibernate's HQL. Alternately, the QueryBuilder classallows one to construct queries through a fluent interface.
Writing queries explicitly however is not always necessary, as Doctrine performs joins and fetches related objects automatically. Small projects can be easily constructed without writing queries.
support for hooks (methods which can validate or modify database input and output) and event listeners to structure business-related logic; column aggregation inheritance (similar objects can be stored in one database table, with one type-column specifying the subtype of the particular object - the correct subclass is always returned when a query is done); a caching framework, making use of several backends such as memcached, SQLite or APC; ACID transactions; database migrations; a "compile" function to combine many PHP files of the framework into one, to avoid the performance hit usually incurred by including the many PHP files of a framework.