Due to high penetration of Distributed Renewable Energy Resources and their inflexible dispatch nature, modern and future electrical grids are facing technical challenges. New participants can offer the flexibility required by the grid through investments in Energy Storage and Demand Response capabilities, however, traditional and Peer-to-Peer business models are insufficient to take advantage of such flexibility. This article provides a comprehensive review on the concept of Energy Communities, their technical and economic motivations, available resources in the literature, trading schemes, and benefits for the grid. Furthermore, this review describes the policy framework in the European Union to make Energy Communities a reality. Moreover, a brief survey of related projects over the world is presented, along with a discussion of future research opportunities in this area.