1. A free SQL server run under the BSD license. From Rute-Users-Guide

2. Object-relational SQL database, descended from POSTGRES. PostgreSQL is an object-relational database, which supports a large part of SQL-92. It is under continuous development and each release implements more of the SQL standard, to the extent that it is now probably more compliant than many commercial databases. It also supports some object-oriented features. As compared to MySQL, PostgreSQL is more fully featured - most importantly, PostgreSQL supports transactions on all tables, something which is essential to multi-user update of a database and referential integrity, which, equally, is essential to a reliable database. The trade-off is that it can be slower, though again, each release sees improvements in efficiency. For more information, see <http://OpenACS.org/why-not-mysql.html>. This package provides the backend features; you need postgresql-client or some other front-end to be able to access them. A package providing ident-server is needed if you want to authenticate remote connections with identd. From Debian 3.0r0 APT

3. PostgreSQL is an advanced Object-Relational database management system (DBMS) that supports almost all SQL constructs (including transactions, subselects and user-defined types and functions). The postgresql package includes the client programs and libraries that you'll need to access a PostgreSQL DBMS server. These PostgreSQLclient programs are programs that directly manipulate the internal structure of PostgreSQL databases on a PostgreSQL server. These client programs can be located on the same machine with the PostgreSQL server, or may be on a remote machine which accesses a PostgreSQLserver over a network connection. This package contains the client libraries for C and C++, as well as command-line utilities for managing PostgreSQL databases on a PostgreSQL server. If you want to manipulate a PostgreSQL database on a remote PostgreSQL server, you need this package. You also need to install this package if you're installing the postgresql-server package. From Mandrake 9.0 RPM