Google advertising business, mobile phones, video games are all example of software platforms. Software platforms are everywhere and being able to understand their business models is essential if you want to play in these domains.
Here a nice quote on the different business models from the highly recommended book Invisible Engines: How Software Platforms Drive Innovation and Transform Industries
When you go to a video game arcade, you have to pay everytime you start a game. At home, once you have bought a game for your video game console, you can play it as often as you like. And if you play online, you can also play as often as you like, but only if you pay a montly fee. When you play music on your computer, you can either use the media playerthat came with it or download others for free and use them instead. RealNetworks sells its digital content by subscription; Apple charges 99 cents per song. Computer manifacturers can pay Microsoft for the right to install Windows (with larger manifacturers receiving a volume discount) or they can install Linux for free. Software developers can pay Sony a royalty for each copy of a PlayStation game they sell or they can get Apple’s help to write game for the Macintosh and pay Apple no royalties at all. Mobile phone users in the United States they are generall charged according to the number of minutes they are connected to the network, with various volume discounts available, while i-mode users in Japan pay according to the amount of data they send or receive, regardless of how long they are connected.
And in the latest issue of Harvard Business Review, April 2009, there is a very good article on business strategies on software and Internet platforms, (subscription might be required):