In this paper, we lay the foundations for a contextualisation of trust, the role it plays, and its different layers within the context of a novel paradigm: Social Cloud Computing. In a Social Cloud, trust plays a vital role as a collaboration enabler. However, trust is not trivial to define, observe, represent and analyse as precursors to understand exactly what role it plays in the enablement of collaboration. We do this through the definition of structure of a Social Cloud as a sequence of social and cognitive processes. We then survey research from the domains of computer science, economics and sociology that consider trust in online communities and exchange scenarios to illustrate the complexity of modelling trust in our scenario. Finally, we define trust within the context of a Social Cloud and identify the core components of trust to facilitate its understanding.