In Salesforce, relationships between objects are crucial to the platform's functionality. Master Detail and Lookup are two types of relationships that define how records can be associated with each other. 1. Master-Detail Relationship: A master-detail relationship is a parent-child relationship in which the master object controls certain behaviors of the detail object. This relationship is tightly coupled: the detail record cannot exist without the master. When a master record is deleted, all related detail records are also deleted. The security and sharing settings of the master record control the detail record. The master-detail relationship allows roll-up summary fields on the master record, where you can perform operations like sum, min, max, avg, etc., on detail record fields. 2. Lookup Relationship: A lookup relationship is a more loose connection between objects. In this relationship, the associated records can exist independently of each other. Deleting a parent record in a lookup relationship doesn't result in deleting the child records. Each object in the relationship has its own security and sharing settings. No roll-up summary fields are allowed in the parent record. To summarize, the master-detail relationship is a more tightly coupled relationship with cascading actions and shared security, while the lookup relationship is more flexible, allowing for independent records and separate security settings.