A basic but comprehensive theory and terminology that applies to all different types of contacts in silicon solar cells has, thus far, been elusive. In this paper, we show that, while there are many desirable characteristics that make a good contact, passivation and low majority-carrier resistance are the two crucial criteria. Since all solar cells have a high density of defects that cannot be reduced at the outer metallic surfaces, limiting minority-carrier transport through the contact remains the only method of passivation. Additionally, when current is extracted from the solar cell, low power loss is achieved through uninhibited majority-carrier transport, which requires a low majority-carrier resistance in the contact. We show that carrier selectivity is automatically achieved when these two criteria are met. Furthermore, we show that selectivity can come from the selectivity of a fabricated layer as well as an induced selectivity in the bulk. We apply these principles to describe a few popular contact technologies and suggest that the same may be done for novel contacts.