PostgreSQL is our most used relational database system in the Lab.
The Wikipedia article for Postgres gives a fairly thorough overview.
One of the higlights of the PostgreSQL community is their wealth of documentation at https://www.postgresql.org/docs/manuals/. Their manual is hence a good read, even though it is 2000+ pages long. Reading that manual will give you a better introduction to Postgres than our documentation could cover.
The PostgreSQL team uses https://git.postgresql.org/gitweb/?p=postgresql.git as their central Git repository.