If your question is explicitly for the PostgreSQL database (server or client). Targeting "any" database, rather use the 'database' tag.
PostgreSQL, most times simply called Postgres, is an open-source object-relational database management system. The term SQL stands for "Structured Query Language", a language commonly used with database systems.
Being open-source, Postgres is available on many platforms. Widely used as it is, it ships in the repositories of most linux distributions. More details on Postgres can be found on the linked Wikipedia page, and also at the project site.