What is “Master-Detail Relationship” in Salesforce?

Master-Detail (1:n)

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.
The master-detail relationship field is required on the page layout of the detail record.
By default, records can’t be reparented 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.
You can define master-detail relationships 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. In addition, you cannot create a master-detail relationship in which the User or Lead objects are the master.