With sharing rules, you are in effect setting automatic extensions to your Organization-Wide Sharing Defaults settings for particular sets of users. As shown in the following screenshot, this can be considered as opening up visibility and access to records for those users.
Sharing rules apply to:
Sharing rules extend the access specified by OWDs and the role hierarchy.
Sharing rules can never be stricter than your OWDs settings and allow wider data access for the included users or groups of users. To define sharing rules, navigate ...