How are sharing rules used in Salesforce?

Hi,
Sharing rules are written to give edit access (public read and write) or public read-only access to certain individuals in Salesforce Org. A classic example is when:- only your managers or superiors need to be given extra credentials to your records in objects as compared to your peers.
By default, all users in your organization will have organization-wide-default sharing settings of either Public Read Only or Private.
To give access to more records, which users do not own, we write sharing rules.
Example: Sharing rules are used to extend sharing access to users in public groups or roles. Hence, sharing rules are not as strict as organization-wide default settings. They allow greater access for those users.