The innovations of sixth generation wireless communication (6G) as compared to fifth generation (5G) are considered in this paper based on the analysis of related works. With the aim of improving multiple communication targets in each service, five 6G core services are identified for different target requirements. Two centricities and eight key performance indices (KPIs) are detailed to describe these services, and enabling technologies are discussed to satisfy the KPIs. A 6G architecture is proposed as an integrated system of the enabling technologies, and is then illustrated using four typical application scenarios. Potential challenges for development of 6G technology is then discussed and possible solutions are proposed. Finally, opportunities for exploring 6G are analyzed in order to guide future research.