Users can manually share certain types of records with other users within the Salesforce CRM application. Some objects that are shared automatically include access to all other associated records. For example, if a user shares one of their account records, then the granted user will also have access to all the opportunities and cases connected to that 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 a part of an Organization-Wide Sharing setting. To be able to grant sharing access for a record, the user must either be the record owner, a system administrator, a user in a role above the owner in the hierarchy, or any user that has been granted full access; or the Organization-Wide settings for that object must allow access through hierarchies.
Users grant access simply by clicking on the Sharing...
