Users can manually share certain types of records with other users within the Salesforce CRM application. Some objects that are shared automatically include permission to the associated records. As an example, should a user share one of their account records, then the granted user also has permission to any opportunities that are connected to the shared account.
Manual sharing rules are generally used either on a one-off basis to share a record or whenever there is difficulty trying to determine a consistent set of users, groups, and the associated rules that would be involved as part of an Organization-Wide Sharing setting. To be able to grant sharing access to records, the user has to be either the owner of the record, a system administrator, assigned to a role above the owner of the record in the role hierarchy, or a user has been permitted full access...