A file with .nsf (Notes Storage Facility) extension is a database file format used by the IBM Notes software, which was previously known as Lotus Notes. It defines the schema to store different kind of objects such like emails, appointments, documents, forms and views. Learn more about this file format here.

OST or Offline Storage Files represent user's mailbox data in offline mode on local machine upon registration with Exchange Server using Microsoft Outlook. Learn more about this file format here.

Files with .PST extension represent Outlook Personal Storage Files (also called Personal Storage Table) that store variety of user information. Learn more about this file format here.