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.