Signed network is a kind of network including edges with the property of positive or negative sign. The positive and negative sign represent positive relationship and negative relationship, respectively. Many real complex networks have opposite relationships, especially in social, biological and information fields. Using the sign properties of edges to analysis, understand and predict the topological structures, functions and dynamic behaviors of these real networks has important theoretical significance and practical applications, such as personalized recommendation, prediction of attitudes, user analysis and clustering and so on. However, academic research has devoted little attention to signed network. This paper reviews the background, significance, research situation and recent progress of signed networks, and discusses the main problems of existing works. This study is to provide a clear and comprehensive understanding to this meaningful research area, and to benefit to the researchers from the fields of network data mining, complex network analysis, sociology and biological information.