ERP stands for "enterprise resource planning", a class of business software. ERP software is designed to integrate information regarding many different aspects of a business, possibly including (among others):
- CRM (contact relationship management)
- order entry
ERP differs from other software in that it tries to integrate several aspects of business management, rather than just one or two.
For further information see: http://en.wikipedia.org/wiki/Enterprise_resource_planning
OFBiz (Open For Business) is a complete open-source ERP implementation.