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The UUID type.

You don't need to do anything additional to insert/update/read uuids in postgres, but if you want to generate them, you'll need to enable the functions with a script from contrib first:
psql -d -U -f /usr/share/postgresql-8.4/contrib/uuid-ossp.sql

Then you can:
select uuid_generate_v4();

You should see a uuid in the output.

The functions available via the uuid-ossp module

Other examples of UUID use:
Alter a table to automatically generate uuids:
alter table my_table alter column id set default uuid_generate_v4();

Create a table to automatically generate uuids:
CREATE TABLE my_table ( id UUID DEFAULT uuid_generate_v4() PRIMARY KEY);

Postgres 9.1