PostgreSQL, also called Postgres, is an open-source object-relational database system: it keeps data in tables of rows and columns, and programs read and change the data with SQL, the standard query language for such databases. It is developed by the PostgreSQL Global Development Group, and its origins go back to the POSTGRES project at the University of California at Berkeley in 1986.
PostgreSQL has been ACID-compliant since 2001: every transaction completes fully or not at all, even after a crash. It uses multi-version concurrency control (MVCC), so readers and writers do not block each other, and write-ahead logging for crash recovery and replication. Besides numbers and text it stores JSON documents, arrays, ranges and geometric types, and it offers several index types, full-text search, and row-level security. Extensions add features such as PostGIS for geographic data, and stored functions can be written in PL/pgSQL, Python, Perl or Tcl.
It is free and open source under the PostgreSQL License, a permissive license similar to BSD or MIT, and runs on all major operating systems.