Besides conventional mobile broadband communication service, 5G is envisioned to support various new use cases from vertical industries. These new scenarios bring diverse and challenging requirements, such as a broader range of performance, cost, security protection, and mobility management. The one-size-fits-all design philosophy applied in existing networks is not viable any more. Slicing a single physical network into several logical networks customized to different unique requirements has emerged as a promising approach to fulfill such divergent requirements in a sustainable way. In this article, we provide a comprehensive survey of 5G network slicing. We first present the driving forces and the concept of network slicing. Then related key enabling technologies, including network function virtualization and modularization, dynamic service chaining, management and orchestration are discussed. The latest progress in 3GPP standardization and industry implementation on 5G network slicing is presented. Finally, the article identifies several important open issues and challenges to inspire further study toward a practical network slicing enabled 5G system.