What is the difference between a Lead and a Contact?

Leads are raw details about individuals or representatives of organizations collected from trade shows, seminars, advertisements, purchasing and such external sources and marketing campaigns. 

Contacts are the representatives of organizations with whom communication is established to pursue a business opportunity. In a typical B2C scenario, contact is the direct point of communication for business-to-customer scenario; whereas in a B2B scenario, contact is the reference point within an organization (business opportunity).