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)
• sales
• order entry
• shipping
• invoicing
• inventory
• purchasing
• accounting

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.