The lookup relationship is a loosely coupled relationship which does not include the rollup summary. It primarily does not require the parent record for the creation of the child record. The master detail relationship is a strongly coupled relationship which includes a roll up summary field. It primarily requires a parent field for the creation of the child record. The master detail field is required on the page layout in master detail relationship while the lookup fields are not required on the page layout in lookup relationship. In the master-detail relationship, there cannot be a child record without a parent whereas there can be a child record without a parent in the lookup relationship. 40 lookups are available on an object in the lookup relationship while two master details are allowed on an object in the master detail relationship.
An external lookup relationship is used for linking a custom, child or an external object to a parent object. The external ID field values present on the parent external object will be matched with the values off the external relationship field.
There are 2 different types of lookup relationships in Salesforce. They are external lookups and indirect lookups.
There can be a maximum of 40 lookups on an object.
Yes it is possible to convert a lookup relationship into master-detail relationship only if the lookup fields in the records contain a value. you can Convert master detail relationship into a lookup relationship as long as there are no roll up summary fields present in the master object