This paper concerns about the conceptualization and categorization of social services in the viewpoint of economics. We mainly explain the economics model for the justification of public provision of social services base on consumption externality, and propose the definition and categories of social services in our policy environment. Consumption externality is to be considered as ‘the antipathy of the society for the quality difference among the consumers’in service markets. The optimal provision mechanism can be justified as an element in the set of cash aid, public provision, mixed provisions, voucher methods, etc. according to the degree of consumption externality and the minimal affordable level of social welfare. We practically define social services as ‘the personal services that have large consumption externality and need public provision in a social perspective’and categorize social services in the broad sense, in the narrow sense, and in the most narrow sense.