The climate change crises, exacerbated by the global dependency of fossil fuels, have brought significant challenges. Extensive renewable-energy-based electrification in the mid- to long-term is considered to be the most promising development pathway to solve these challenges. However, this is tangible only if the energy infrastructure can accommodate renewable energy sources and distributed energy resources, such as batteries and heat pumps, without adversely affecting power grid operations. To realize renewable-energy-based electrification goals, a new concept—the Energy Internet (EI)—has been proposed, inspired by the most recent advances in (data) information and telecommunication network architectures. Recently, many measures have been taken to practically implement the EI as well. Although the different EI models share many ideas, a definitive universal definition of the EI is currently unavailable. Moreover, some studies have proposed protocols and architectures, but a generalized technological overview is still missing. Such an understanding of the technologies that underpin and encompass the current and future EI is very important to push toward a standardized version of the EI that will eventually make it easier to implement the EI across the world. In this paper, we first examine and analyze the typical popular definitions of the EI in scientific literature. Based on their definitions, assumptions, scope, and application areas, the papers are then classified into four different groups representing the way in which the papers have approached the EI. Then, we synthesize these definitions and concepts and keeping in mind the future smart grid, we propose a new universal definition of the EI. We also identify the underlying key technologies for managing, coordinating, and controlling the multiple (distributed or not) subsystems with their own particular challenges. We conclude this survey by highlighting the main open challenges and requirements in terms of system complexity, security, standardization, energy trading and business models, and social acceptance in order to have an EI-based energy system in the future.