Freemium is a business model by which a product or service (typically a digital offering such as software, media, games or web services) is provided free of charge, but a premium is charged for advanced features, functionality, or virtual goods.[1][2] The word “freemium” is a portmanteau combining the two aspects of the business model: “free” and “premium”.
http://en.wikipedia.org/wiki/Freemium

So there you go.