What are the differences between sap & ERP? is it convenient 4 the user of erp 2 use the sap?If ERP part of SAP so y companies need specially sap users?

your question is not very clear as SAP is an ERP, companies need SAP user as it is the best ERP and SAP user are in great demand

Openerp Evaluation with SAP as Reference

SAP is an ERP and used by a lot of the500 fortune companies and other mega ones around the world. And, yes, you will need SAP trained users to effectively customize it to match your needs and effeceintly use it to boost your productivity and maintain your competitivness.

I fully agree with all answers