Lookup Vs Master Detail relationship

1.No, you cannot have a rollup summary field that uses a Lookup relationship. it must be on the parent of the master/detail relationship.

Lookup Relationships:


1. Loosely coupled
2. Child field value may be optional
3. No cascade delete
4. Independent ownership & sharing
5. Maximum 25 lookup relationships per child


Master/Detail Relationships:


1.Tightly coupled
2.Child field value is always required
3.Cascade delete
4.Inherited ownership & sharing
5.Maximum 2 master relationships per detail object
6.Can be multiple levels deep
7.If master is a standard object: 2 additional subdetails
8.If master is a custom object:  3 additional subdetails