What is Apex Managed Sharing and when is it needed?

Apex Managed Sharing is used when sharing requirements are too complex for declarative sharing rules. Each custom object has a corresponding Share object (e.g., MyObject__Share). An Apex class (typically a trigger or batch job) creates records in this Share object to grant specific users or groups access to specific records. Key fields on the Share object: ParentId (the record being shared), UserOrGroupId (who receives access), AccessLevel (Read or Edit), RowCause (must be a custom sharing reason created in Setup — this differentiates Apex Managed Sharing from manual sharing and prevents deletion during sharing recalculation). Apex Managed Sharing rows with custom row causes persist through sharing recalculations.