Can triggers update other tables?

VERSION = 'PostgreSQL 8.4.2 on i686-pc-linux-gnu, compiled by GCC gcc
(GCC) 3.4.6, 32-bit'

I'm reading at http://www.postgresql.org/docs/8.4/static/sql-createtrigger.html
and http://www.postgresql.org/docs/8.4/static/plpgsql.html but I
cannot solve this problem. The very outline is that I'm having two
tables (in reality there are a lot more), something like this:

SUMMARY ( id integer, num_stores integer; );
ITEMS ( id serial PRIMARY KEY; name text; some_reference integer,
some_value smallint; ).

When a row is inserted in ITEMS, I want to launch a trigger that
issues a "UPDATE num_stores SET num_stores=(SELECT count(*) FROM items
WHERE id = <trigger_table.id>) where id = <trigger_table.id>".
Triggers seem to be run on functions returning plpgsql and I don't
know how to issue ordinary SQL-commands in SQL, so I don't know how to
reference these two tables in a single trigger.

Is it possible? If yes, which section of the manual is the place for
me to read up on?