Microservices are emerging as a new computing paradigm which is a suitable complementation of cloud computing. Microservices will decompose traditional monolithic applications into a set of fine-grained services, which can be independently developed, tested, and deployed. However, there are many challenges of microservices. This paper provides a comprehensive overview of microservices. More specifically, firstly, we systematically compare traditional monolithic architecture, service-oriented architecture (SOA), and microservices architecture. Secondly, we give an overview of the container technology. Finally, we outline the technical challenges of microservices, such as performance, debugging and data consistency.