Master – Detail Relationship :

1. If we delete master records then detail (Child) records are deleted.
2. It creates the parent(master) child(Detail) relationship between objects.
3. Child records do not have Owner fields it takes parents records owner as child records owner.
4. We can have maximum 2 master details on an object
5. Roll up summary can be made in Master - child relation
6. Standard objects can never be on the detail side of the master detail relationship 
7. parent is required field while creating child record.

Look up relationship :

1. Look up relationship creates relations between two objects.
2. If we delete any object then other object is not deleted.
3. We can have maximum 25 lookup on an object.
4. Roll up summary is not valid.
5. standard objects'detail side not possible of the master detail relationship .
6. Parent is not required while creating child record.