The paper defines the notion of social signal, in terms of a cognitive model of mind and social interaction, as a communicative or informative signal or a cue that directly or indirectly provides information about "social facts": social interactions, social emotions, social attitudes, evaluations and stances, social relations, and social identities. This notion is compared with other connected notions in Semiotics, Ethology, and Psychology, several types of informative and communicative signals and cues are exemplified, and their process of production and interpretation is overviewed, while considering the role of context and previous knowledge in it. The implications of this model are outlined for the construction of systems for the analysis of social signals and their simulation in Virtual Agents.