At a basic level, the Web is an application developed to let people share information over the Internet. We can view the original conception and design of the Web as involving two central features. First, it provided a way for you to make documents easily available to anyone on the Internet, in the form of Web pages that you could create and store on a publically accessible part of your computer. Second, it provided a way for others to easily access such Web pages, using a browser that could connect to the public spaces on computers across the Internet and retrieve the Web pages stored there.

This is still how we experience the Web today: as a sequence of Web pages rendered inside a browser. For example, Figure shows a set of four separate Web pages: the home page of a college instructor who teaches a class on networks; the home page of the networks class he teaches; the blog for the class, with a post about Microsoft listed at the top; and the corporate home page for Microsoft. Because of the underlying design, we can think of these pages both as part of a single coherent system (the Web), but also as files that likely reside on four separate computers, controlled by several different and completely independent organizations, and made publically accessible by a now-universal consensus to participate in the protocols of the Web.

The Bow-Tie Structure of the Web:

1. It was invented in 1991 by Andrei Brodal et al. This structure represents a global map of the web using the Strong Connected Components (SSCs) as the basic building blocks (component graph). It divides the web into a few large pieces and show how they fit together.
2. The web contains giant SSC and this giant SSC contains a significant fraction of all pages (also most important pages: major commercial, governmental, and non-profit organizations).

Next step is to position all the remaining SSCs in remaining SSCs in relation to the giant one. This involves classfying nodes by their ability to reach and be reached from the giant SCC. The first two sets in this classification are the following. (1) IN: nodes that can reach the giant SCC but cannot be reached from it — i.e., nodes that are “upstream” of it. (2) OUT: nodes that can be reached from the giant SCC but cannot reach it — i.e., nodes are “downstream” of it. To better understand IN and OUT , let’s take example of university:

IN:

1. “I am student at Univ. of X “
2. “I am applying to college Institute”

OUT:

1. “Blog post about Company Z ”
2. SCC involving company Z.

The figure shows the relation of IN, OUT, and the giant SCC. Because of the visual effect of IN and OUT as large lobes hanging off the central SCC, Broder et al. termed this the “bow-tie picture” of the Web, with the giant SCC as the “knot” in the middle.

As Figure also makes clear, there are pages that belong to none of IN, OUT, or the giant SCC — that is, they can neither reach the giant SCC nor be reached from it. These can be classified be further classified as:

1. Tendrils: The “tendrils” of the bow-tie consist of (a) the nodes reachable from IN that cannot reach the giant SCC, and (b) the nodes that can reach OUT but cannot be reached from the giant SCC. For example, the page My song lyrics in Figure 1is an example of a tendril page, since it’s reachable from IN but has no path to the giant SCC.
2. Tubes : It’s possible for a tendril node to satisfy both (a) and (b), in which case it’s part of a “tube” that travels from IN to OUT without touching the giant SCC. (For example, if the page My song lyrics happened to link to Blog post about Company Z in Figure 1, it would be part of a tube.)

Disconnected: Finally, there are nodes that would not have a path to the giant SCC even if we completely ignored the directions of the edges. These belong to none of the preceding categories.

While the bow-tie picture gives us a global view of the Web, it doesn’t give us insight into the more fine-grained patterns of connections within the constituent parts — connections which could serve to highlight important Web pages or communities of thematically related pages.