If you experience problems, then you'll need to double-check which version of the server you have. This will help you report a fault or to consult the correct version of the manual.
Connect to the database, and then issue the following:
postgres # SELECT version();
You'll get a response with something like the following:
PostgreSQL 9.0 on x86_64-unknown-linux-gnu, compiled by GCC gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3, 64-bit
That's probably too much information all at once!
PostgreSQL server version's format is Major.Minor.Maintenance
In some other software products, the Major release number is all you need to know, but with PostgreSQL the feature set and compatibility relates to the Major.Minor ...