Master-Detail Relationship in Salesforce is a parent-child relationship in which the master object controls certain behaviors of the detail object.

• When a record of the master object is deleted, its related detail records are also deleted.
• The Owner field on the detail object is not available and is automatically set to the owner of its associated master record. Custom objects on the detail side of a master-detail relationship cannot have sharing rules, manual sharing, or queues, as these require the Owner field.
• The detail record inherits the sharing and security settings of its master record.
• It is required on the page layout of the detail record.
• By default, records can’t be re-parented in master-detail relationships. Administrators can, however, allow child records in master-detail relationships on custom objects to be reparented to different parent records by selecting the Allow reparenting option in the master-detail relationship definition.
• It can be defined between custom objects or between a custom object and a standard object. However, the standard object cannot be on the detail side of a relationship with a custom object.
• The data related to the object appears on a related list.
• When we undelete the master record then all its related details records also get undeleted.

Note: Master-Detail relationship cannot be created where User or Lead objects are the master.

Next up is Lookup Relationship.