Explanation
The Account and Contact objects have a lookup relationship. A lookup relationship is a type of relationship that links two objects together, but does not affect security, ownership, or deletion. A lookup relationship allows the user to select a record from another object and link it to the current record. For example, the Account field on the Contact object allows the user to select an account that the contact is related to. A self relationship is a type of lookup relationship that links an object to itself, creating a hierarchy of records. For example, the Reports To field on the Contact object allows the user to select another contact that the contact reports to. A master-detail relationship is a type of lookup relationship that creates a parent-child relationship between two objects, where the child records inherit the security and ownership of the parent record, and are deleted when the parent record is deleted. For example, the Account field on the Opportunity object is a master-detail relationship that links the opportunity to the account.