This document discusses the freemium business model, where core products are offered for free while premium products and features are paid. It provides examples like Skype and LinkedIn that offer basic free services but make money through optional upgrades. The document outlines how freemium differs from traditional paid models and when freemium works best, such as when a quality free product can be duplicated digitally at low cost to reach a large audience. Premium upgrades are then sold to support the free offerings and generate revenue.