Because search engines are like nosy know-it-alls that just want to scan through your Rolodex to see who you know and why they should think you’re worth their time. You are the friends you keep, so make sure you’re keeping good company.

In This Sub-Section of the article on Link Graphs and Information Retrieval, I’ll cover types of Link Graphs and how graphs are used by search engines.

**What is a Link Graph of the Web?**

A graph is a map that shows how different objects are connected to each other.

A graph is a formal mathematical representation of a network (a collection of objects connected in some fashion).

Each connection in the graph is called a **node**. Corresponding to the connections (or lack thereof) in a network are **edges** in a graph. Each edge in a graph has two distinct nodes.

The image above shows a **graph** where the colored circles numbered 1, 2, 3, 4, 5 are nodes that are connected to each other. The dotted lines are the edges connecting the nodes in the graph.

**A graph is how the internet is organized by search engines**. Google specifically uses its index (model of the web as a graph) to categorize and prioritize documents available on the web.

Essentially, understanding what a graph is and how it works is fundamental to understanding more advanced topics such as crawling, indexing and link building.

**Types of Link Graphs:**

Now that you’re familiar with the graph, let’s see how graphs are used by search engines. There are two types of graphs and the main difference between them is the type of connections between the nodes in the graph. Directed and undirected graphs are used by search engines to index, categorize and evaluate the documents available on the web.

**Directed graph**

A directed graph has edges that are linked to specific nodes e.g. a directed graph of three nodes A, B and C will look like this.

Node A is separately connected to Node B and Node C through two separate directed edges. If you’re at Node A, you can move to Nodes B or Node C but you can’t get back to Node A from either of the two nodes.

While Nodes B and Node C are connected to each other allowing you to move back and forth between the two nodes but you can’t go to Node A from either of the two nodes.

**Undirected graph**

As you would assume, information available on the internet is connected without restrictions. A graph of nodes and edges with multiple links with each other is called an undirected graph.

In an undirected graph, each node is connected to every other node on the same graph either through an undirected edge or through another node that acts as a link between the two nodes.

**How link graphs are used by search engines**

Both directed and undirected graphs are usedorganizingsing and evaluating the quality of the information available on the internet. Here’s the basic premise of link analysis in evaluating the relevance and quality of information available on a webpage (node):

**A: If Page A is linked to Page B (unless Page B is the homepage) then the content on Page B must somehow add value to the content on Page A. This might also signify the ****similarity in the topic of the information available on pages A and B.**

**B: If more pages e.g. Page C and Page D also link to page B then this signifies the importance of the information available on Page B to the topic that is the subject of the content on pages C and D.**

That’s the simplest way to understand how search crawlers understand the ** topical relevance** and

**of information available on the internet by using graphs.**

*evaluate the quality***Understanding the web through link graphs**

The information on the internet is organised in the form of graphs that range from providing organisation based on links, to providing information about citation, social relationships between entities and connections between entities and their attributes:

- Link Graph
- Co-Citation Graph
- Social Graph
- Knowledge Graph

A **link graph** is a directed relationship between documents that are connected with each other via hyperlinks. Link graphs are used in ranking different documents available on the internet.

A **co-citation graph** on the other hand is undirected. Co-citations among documents help determine the topical relevance and categorisation of these documents.

A **social graph** displays a network of implicit and explicit relationships between individuals on the web. You can see this type of graph at work on social media platforms like Facebook (your friends and people you may know) and on LinkedIn (a tiered categorisation of how and through which individuals you are connected to other people).

A **knowledge graph** shows relationships between entities (e.g. Christian Bale) and facts related to that entity available across different categories on the internet (e.g. facts about his personal life, his professional accomplishments, roles he’s played, films he’s acted in etc.).

Hope it was USEFUL,

Blessings,