PostgreSQL is an (ORDBMS) object-relational database management system that emphasis on standards-compliance and extensibility. Its primary function as a database server is storing data, securely and supporting best practices, and retrieving it later, as requested by other software application let it be those running on another computer across a network or those on the same computer.

Workloads ranging from small single-machine applications to large Internet-facing applications can be handled by it with a number of concurrent users. Replication of the database itself is also provided by recent versions for availability and scalability.

PostgreSQL evolved at the University of California from the Ingres project, Berkeley by the leader of the Ingres team, Michael Stonebraker. In 1982, he left Berkeley for making a proprietary version of Ingres. PostgreSQL consists of a built-in support for regular hash indexes and B-tree, and two types of inverted indexes: generalized inverted indexes (GIN) and generalized search trees (GiST). 

PostgreSQL handles complex SQL queries using many indexing methods not available in other databases; has materialized views and updateable views, triggers, foreign keys; supports stored procedures and functions and other expandability; has a number of extensions that are written by third parties. In addition to the probability of working with the open source databases and major proprietary, PostgreSQL supports migration from them by its available migration tools and extensive standard SQL.

PostgreSQL runs on many operating systems including FreeBSD, Linux, Solaris, and Microsoft Windows. Starting with OS X 10.7 Lion, Mac OS X has PostgreSQL client tools in the desktop edition and has the server as its standard default database in the server edition. The Linux distributions vast majority have it available in supplied packages.
It is open source software and free, released under the PostgreSQL License terms, a permissive free software license.