Where does PostgreSQL store the database?

On Windows7 all the databases are referred by a number in the file named pg_database under C:\Program Files (x86)\PostgreSQL\8.2\data\global. Then you should search for the folder name by that number under C:\Program Files (x86)\PostgreSQL\8.2\data\base. That is the content of the database.

Under my Linux installation, it's here: /var/lib/postgresql/8.x/

You can change it with initdb -D "c:/mydb/"

The location of specific tables/indexes can be adjusted by TABLESPACEs:

CREATE TABLESPACE dbspace LOCATION '/data/dbs';
CREATE TABLE something (......) TABLESPACE dbspace;
CREATE TABLE otherthing (......) TABLESPACE dbspace;

picmate's answer is right. on windows the main DB folder location is (at least on my installation)

C:\PostgreSQL\9.2\data\base\

and not in program files.

his 2 scripts, will give you the exact directory/file(s) you need:

SELECT oid from pg_database WHERE datname = <database_name>;
SELECT relname, relfilenode FROM pg_class WHERE relname = <table_name>; 

mine is in datname 16393 and relfilenode 41603

I'm running postgres (9.5) in a docker container (on CentOS, as it happens), and as Skippy le Grand Gourou mentions in a comment above, the files are located in /var/lib/postgresql/data/.

$ docker exec -it my-postgres-db-container bash
root@c7d61efe2a5d:/# cd /var/lib/postgresql/data/
root@c7d61efe2a5d:/var/lib/postgresql/data# ls -lh
total 56K
drwx------. 7 postgres postgres   71 Apr  5  2018 base
drwx------. 2 postgres postgres 4.0K Nov  2 02:42 global
drwx------. 2 postgres postgres   18 Dec 27  2017 pg_clog
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_commit_ts
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_dynshmem
-rw-------. 1 postgres postgres 4.4K Dec 27  2017 pg_hba.conf
-rw-------. 1 postgres postgres 1.6K Dec 27  2017 pg_ident.conf
drwx------. 4 postgres postgres   39 Dec 27  2017 pg_logical
drwx------. 4 postgres postgres   36 Dec 27  2017 pg_multixact
drwx------. 2 postgres postgres   18 Nov  2 02:42 pg_notify
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_replslot
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_serial
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_snapshots
drwx------. 2 postgres postgres    6 Sep 16 21:15 pg_stat
drwx------. 2 postgres postgres   63 Nov  8 02:41 pg_stat_tmp
drwx------. 2 postgres postgres   18 Oct 24  2018 pg_subtrans
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_tblspc
drwx------. 2 postgres postgres    6 Dec 27  2017 pg_twophase
-rw-------. 1 postgres postgres    4 Dec 27  2017 PG_VERSION
drwx------. 3 postgres postgres   92 Dec 20  2018 pg_xlog
-rw-------. 1 postgres postgres   88 Dec 27  2017 postgresql.auto.conf
-rw-------. 1 postgres postgres  21K Dec 27  2017 postgresql.conf
-rw-------. 1 postgres postgres   37 Nov  2 02:42 postmaster.opts
-rw-------. 1 postgres postgres   85 Nov  2 02:42 postmaster.pid

A single Terminal command: pg_lsclusters, (using Ubuntu)

What you need is under Data directory:

Ver Cluster Port Status Owner    Data directory               Log file
10  main    5432 online postgres /var/lib/postgresql/10/main  /var/log/postgresql/postgresql-10-main.log
11  main    5433 online postgres /var/lib/postgresql/11/main  /var/log/postgresql/postgresql-11-main.log