What's the difference between intranet, extranet and internet?

• Intranet is shared content accessed by members within a single organization.
• Extranet is shared content accessed by groups through cross-enterprise boundaries.
• Internet is global communication accessed through the Web.

For better comprehension, take a look at this drawing:

Summary:

The Internet, extranets, and intranets all rely on the same TCP/IP technologies. However, they are different in terms of the levels of access they allow to various users inside and outside the organization and the size of the network. An intranet allows for restricted access to only members of an organization; an extranet expands that access by allowing non-members such as suppliers and customers to use company resources. The difference between the Internet and extranets is that while the extranet allows limited access to non-members of an organization, the Internet generally allows everyone to access all network resources.

Not necessarily definitive definitions, but Wikipedia defines them as follows.

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.

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.

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.