Please advise what is the major difference between application "sales" and application "order manage

We have one guy setup the custom order management responsibility using sales application, while this is non-updatable field after saved the record. we only using sales order( including i/c sales order, sales agreement etc basic functions. does anybody could advise us  what is the major difference between application "sales" and application "order management", if there is no big difference, then we will just leave as it is. suppose any huge functions difference, we will disable these responsibilities and create the new one using "order management" applications.