Up to this point in the book, we've considered networks in which the basic units being connected were people or other social entities, like firms or organizations. The links connecting them have generally corresponded to opportunities for some kind of social or economic interaction.

In the next several chapters, we consider a different type of network, in which the basic units being connected are pieces of information, and links join pieces of information that are related to each other in some fashion. We will call such a network an information network. As we will see, the World Wide Web is arguably the most prominent current example of such a network, and while the use of information networks has a long history, it was really the growth of the Web that brought such networks to wide public awareness.

While there are basic differences between information networks and the kinds of social and economic networks that we've discussed earlier, many of the central ideas developed earlier in the book turn out to be fundamental here as well: we'll be using the same basic ideas from graph theory, including short paths and giant components; formulating notions of power in terms of the underlying graph structure; and even drawing connections to matching markets when we consider some of the ways in which search companies on the Web have designed their businesses.

Because the Web plays such a central role in the modern version of this topic, we begin with some context about the Web, and then look further back into the history of information networks that led up to the Web.