PostgreSQL is 100 percent open source software.
PostgreSQL is freely available to use, alter, or redistribute in any way you choose. Its license is an approved open source license, very similar to the Berkeley Distribution Software (BSD) license, though only just different enough that it is now known as The PostgreSQL License (TPL).
PostgreSQL is already being used by many different application packages, so you may find it already installed on your servers. Many Linux distributions include PostgreSQL as part of the basic installation, or include it with the installation disk.
One thing to be wary of, is that the version of PostgreSQL included may not be the latest release. It will typically be the latest major release ...