A large number of network function devices are deployed in enterprise networks and data centers to implement related application functions, improve network performance, and strengthen network security. Most of these network function devices are based on hardware, and have problems such as functional hardening, poor scalability, and unified management difficulties. In order to solve the above problems, both the industry and academia have turned their attention to the network function virtualization (NFV) technology. By decoupling network functions from the physical devices on which they run, network functions are not constrained by physical devices, facilitating the upgrades and updates of  network device services. At the same time, NFV provides possibility for new architectures, systems, and applications. We first introduce the NFV technology and compare it with cloud computing and SDN. Then we elaborate current research results from six dimensions, which are virtual network function (VNF) system architecture, data plane, control plane, deployment mode, implementation language and application. Finally, we summarize and provide an outlook of future NFV research direction.