What is in PostgreSQL?

PostgreSQL is a relational database management system (RDBMS).

PostgreSQL is an object-relational database management system (ORDBMS) PostgreSQL 8.1.5 Documentation
PostgreSQL is an open-source descendant of this original Berkeley code. It supports a large part of the SQL standard and offers many modern features:

• complex queries
• foreign keys
• triggers
• views
• transactional integrity
• multiversion concurrency control

Also, PostgreSQL can be extended by the user in many ways, for example by adding new
• data types
• functions
• operators
• aggregate functions
• index methods
• procedural languages

And because of the liberal license, PostgreSQL can be used, modified, and distributed by everyone free of charge for any purpose, be it private, commercial, or academic.