Intranet –

An intranet is a private computer network that uses Internet Protocol technologies to securely share any part of an organization’s information or operational systems within that organization. Intranets can be used to publish company policies and newsletters, provide sales and marketing staff with product information, provide technical support and tutorials, and just about anything else you can think of that fits within the standard Web server/Web browser environment

Extranet –

An extranet is a private network that uses Internet protocols, network connectivity. An extranet can be viewed as part of a company’s intranet that is extended to users outside the company, usually via the Internet. Extranets are a powerful tool because they let businesses share resources on their own private networks over the Internet with suppliers, vendors, business partners, or customers.

Extranets are typically used for supporting real-time supply chains, for enabling business partners to work together or to share information such as catalogs with customers.

Internet –

The Internet is a global system of interconnected computer networks that use the standard Internet Protocol Suite (TCP/IP) to serve billions of users worldwide.The network formed by the cooperative interconnection of a large number of computer networks.
• Network of Networks
• No one owns the Internet
• Every person who makes a connection owns a slice of the Internet.
• There is no central administration of the Internet.