Look up relationship:
- Parent field on child can be required or optional choice.
- No impact on security and access.
- If we delete parent record then child records are not deleted.
- No limit to number of layers.
- Up to 25 allowed for an object.
- We can change parent object for child object.
- Roll-up summary field is not allowed.
Master – Detail Relationship:
- Parent field on child is required.
- Access to parent determines access to children.
- Deleting parent automatically delete children.
- Limited number of layers.
- Up to 2 allowed for an object.
- We can’t change the parent object for child object once create. The only way is killing the both objects.
- Roll-up summary field is allowed.