MRP: ERP started with Materials Requirement Planning (MRP). MRP catered to the two basic functions of Materials Management and Inventory Management and typically catered to a manufacturing organisation.
MRP-II: An upgrade over MRP was MRP-II. MRP-II provided for production Planning along with Materials Management and Inventory Management.
ERP: Enterprise Resource Planning. This package provided for the complete suite of applications ranging from Sales and distribution, Planning, Materials Management, Quality Management, Human Resources and Financial Accounting.
The keyword with ERP is "Integration".
