PostgreSQL is a
SQL-using
RDBMS that evolved from
Ingres, an ancient UNIX RDBMS (that later evolved into a proprietary program).
It is more robust than some other freely available or open source RDBMSes (has transactions and referential integrity constraints), but, as one reader of sfnet.atk.sodat put it, it has had a reputation of being "slow like an old cow". However, it has been reported that version 7.1 of PostgreSQL is approximately as fast as MySQL already, and it has more features too.
A commendable choice.
It has interfaces to many programming languages, such as C, Perl (via "native" Pg module and DBI with DBD::Pg), Java (via JDBC) and PHP. The latter three guarantee it's a good choice for running dynamic web sites with Apache.