Standard objects can be on the detail side of a custom object in a Master-Detail relationship.

Master-Detail relationships cannot be converted to a look-up relationship.

Deleting a master record in a Master-Detail relationship deletes all related detail records.

Master-Detail relationships can convert to a lookup relationship if no roll-up summary fields exist on the master object.

A Master-Detail relationship cannot be created if the custom object on the detail side already contains data.