All tables that starts with HZ_ are related to the Trading Community Architechture (TCA). In these table we can find all the information about the Customers, Organization, Person's.
From release 11i TCA came into picture in Accounts Recievable module, where oracle has grouped all the customer information at one place. Most important tables in TCA are HZ_PARTIES, HZ_CUST_ACCOUNTS_ALL, HZ_CUST_ACCT_SITES_ALL, HZ_CUST_SITE_USES_ALL, HZ_LOCATIONS, HZ_PARTY_SITES, HZ_PARTY_SITE_USES, HZ_CONTACT_POINTS. few to name.