Zones represent distinct logical pathways (URLs) to the same Web application. Zones allow you to set distinct access and policy requirements for various groups of users. There are a maximum of five zones per Web application. 

Following are the different types of zones available: 

• Intranet
• Internet
• Custom
• Extranet
• Extranet

The Default zone is generated when you create a Web application. The Web application can then be extended to create additional zones. Per Web application, each zone can only be selected once. In a Web application, for example, there can only be one Default zone. In Internet Information Services (IIS), each zone is represented by a separate Web site. The key distinction is that after extending SharePoint, each zone has its own IIS Website.

These zones have unique URLs assigned to them. Each zone has its own port number and protocol (HTTP or HTTPS). In each zone, several AUTHENTICATION methods can be USED to log in. For example, intranet forms authentication, default Windows authentication, and so on. For each zone, you configure your BLOB and Distributed cache differently. 

For example - Assume you have a building, which is your Web Application, with a default zone as its MAIN entrance and its own authentication service, such as Windows authentication in the front for SharePoint administrators, and adding other zones is similar to adding a few additional entrances to your Web Application with Facebook as the identity provider for your FEDERATED partners.