In simple terms, MBA is a PG Degree course in management, whereas, a PGDM is a diploma course in management. Further, if you are looking to pursue a PGDM in distance or online mode, ensure the validity of the University and their approvals.
Moreover, there are certain universities that offer PGDM course with 1 year or 1.5 years as equivalent to a 2 year MBA course. They are NOT the same.