Energy Internet (EI) envisions a future energy system with sustainable concerns of efficiency, economy and environment by achieving flexibility of multi-energy-integrated physical space, digitalization of data-driven cyber space and interaction of customer-aware social space. To systemically understand how EI manages energy, data and information flows, a comprehensive investigation from multi-angle perspectives is presented in this paper. The core building blocks are categorized into three-layered framework: energy-oriented network, communication-oriented network and service-oriented management. Furthermore, this paper provides a multi-disciplinary portrait of today's EI in terms of architecture, technologies, standards, services and platforms. Finally, open issues, future trends and challenges for development of sustainable EI are discussed.