Along with the rising energy consumption in the world, new technologies will influence the way we generate, distribute and consume energy. Alongside others, distributed energy generation becomes a major trend in many countries. The fast upward penetration of Distributed Energy Resources (DER) and the ongoing trend towards a more competitive electricity market requires new technologies and policies to handle both emerging technical and economic issues. In order to handle distributed generation and to intensify its visibility within power markets, the idea of virtual power plant (VPP) has emerged and is used by many researchers. It is composed of combining various small size distributed generating units to form a "single virtual generating unit" that can act as a conventional one and capable of being visible or manageable on an individual basis. Present paper tries to sheds light on main issues related to the idea of the VPP. The paper will review various definitions and primary concept of VPP, its main components, and finally two principal VPP types namely commercial VPP (CVPP) and technical VPP (TVPP).