The article considers the contents of the social network as a scientific category, its relationship to such definitions as «trust» and «social capital». The recent definitions are used by scientists studying informal social actions. The author studies the attributes of social network (rootedness, relatedness and reciprocity) and also different kinds of social network, which are allocated by Russian scientists (whole and personal, weak and strong, family-related and communal). The main theoretical and methodological approaches to the study of social network as a sociological category are: structure functional analysis, symbolic interactionism and R. Emerson’s integrative theory of exchange. The application of structure functional analysis allows describing a structure and functions of social network. The use of symbolic interactionism shows the role of the system acting as a superstructure determining the everyday activities of the social participants. The adaptation of integrative theory of exchange focuses attention on exchange, which allows people to regulate social actions according to value-normative system accepted in social network, social group and society. The author emphasizes the heuristic potential of social network as a sociological category, which can expand conception about informal social actions.