What is Organisation-Wide Default (OWD) and what are its settings?

OWD (Organisation-Wide Defaults) sets the baseline record-access level for all users on an object. Settings:

1. Private: Only the record owner and users above them in the role hierarchy can view/edit.
2. Public Read Only: All users can view but only the owner (and hierarchy) can edit.
3. Public Read/Write: All users can view and edit.
4. Controlled by Parent: The detail record inherits access from its master (used in master-detail relationships). OWD only restricts access — you then open up access using sharing rules, role hierarchy, or manual sharing. You cannot restrict below OWD using sharing.