Owner-based and criteria-based sharing rules
Key Takeaway
Sharing rules can only open access wider than the org-wide defaults. Use an owner-based rule to share records by who owns them, and a criteria-based rule to share records by their field values.
When the org-wide defaults keep records locked down, sharing rules automatically give groups of users extra access to those records. The two types differ in what they look at: the record's owner, or the field values on the record itself.
Sharing rules and org-wide defaults
Sharing rules can only open access wider than the org-wide defaults; they can never restrict access below them. To create a sharing rule on an object, that object's org-wide default must be Public Read Only or Private.
A sharing rule applies to all new and existing records that match it. The rule applies to both active and inactive users.