· You can't create a record without including an M-D relationship field. It is a required field. Lookups are not required fields.
· Cascade deletes - When you delete the parent of a master-detail relationship, all of the child records get deleted too
· Rollup Summary fields are only available on master-detail relationships, not lookups.
· Master Relationship Parent Records inherit the child records but not in Look ups.
· One other important difference is that in Master-Detail, it is the master that controls the ownership/visibility of the child record - the child record doesn't have a separate owner record, so can only be shared via the Parent's visibility rules...but not in Lookup.