Hi Manoj.
If you take the example of the Java programming Language. You have the application wrote in Java and to run it you need to install Java Runtime Enviroment on the computer or Java Development Kit to develope applications.
SAP Applications are developed in ABAP which is an SAP programming language, SAP Netweaver provides the ABAP runtime environment and development environment. It also provides other features such as user management, resource management such as memory allocation to the work processes which execute the ABAP programs which are akin to Java theads.
SAP ERP is a suite of ABAP applications which provide one or more Business functions such as HR, Finance etc. So in short SAP Netweaver provides the underlying Runtime environment and SAP ERP is an collection of ABAP business applications which run in the runtime environment it provides.
If you need any further clarification please ask.
Regards
Neil Ryan
SAP Active Global Support