Setting Permissions for web viewing

What's the cleanest way to set permissions in postgres so that a web
application can properly view and query a back-end database, but not modify?

I have tried modifying pg_hba.conf, and got it to do a pg_connect()
successfully, however, the subsequent pg_Exex() call failed with the
following browser error: "Warning: PostgreSQL query failed: ERROR:
table_name: Permission denied..."

Thanks!

Kevin