In practice, a series of views that organize this data are how most people access the statistics collector's output. Reading the SQL source code to these views is quite informative for learning how they can be used. If you have a PostgreSQL source code tree, the file that creates these views is src/backend/catalog/system_views.sql; in an installed database binary tree, you'll find them at share/system_views.sql; and alternately you can download the source code at http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql.
A simple view from there that shows how these are commonly constructed is the pg_stat_bgwriter view introduced in version 8.3:
CREATE VIEW pg_stat_bgwriter AS
SELECT
pg_stat_get_bgwriter_timed_checkpoints() AS checkpoints_timed,
pg_stat_get_bgwriter_requested_checkpoints(...