What is the difference between a Sharing Set and a Share Group?

Sharing Sets grant external (community) users access to records based on a field relationship — for example, if a Case's Account field matches the user's Account, the user gets access (Read or Edit) to that Case. Sharing Sets are defined in Community settings and apply automatically without creating individual sharing records. They are scoped to specific profiles and objects. Share Groups are public groups that automatically include all users with a specific community profile. A Share Group can then be used in a standard sharing rule to grant those users access to records — for example, "Share all Cases in Status=Open with the Share Group of all Partner users." Share Groups offer more flexibility since they work with the full sharing rule engine, but they grant access to all matching records rather than just records related to the user's account.