Hi Shamil,
NetWeaver in the most basic terms is a computing platform which supports various applications provided by SAP such as ERP, CRM, SRM etc.
It is safe to assume that most large scale enterprises depending on SAP would be utilizing some or most of the applications provided by SAP such as mobililty, CRM, BI, HCM etc. Apart from interaction between these applications, the system might require interaction with external ( non-sap ) applications. At times this might get complicated with all the clustering.
SAP NetWeaver therefore is a common platform which provides internal integration as well as integration with external legacy systems. Developing products over this platform is feasible.
Hope this helps.