With the development of renewable energy and smart grids techniques, energy prosumers, a new class of end consumers, behave more actively and flexibly in energy optimization and management. They have great influence on promoting the community energy efficiency, increasing the energy economics, and improving the stability of local distribution network. In this review, several common types of energy prosumers are introduced firstly along with their characteristics, and their flexibilities and potential values to energy optimization on demand side in smart grids are evaluated. Typical frameworks of community energy sharing are reviewed and their basic features are summarized, along with their advantages and limitations. Based on the analysis, certain closely related problems in energy sharing are discussed, including energy data prediction, equilibrium analysis for games, and distributed optimization. Finally, challenges in energy sharing problems are concluded to throw some light on the future research.