The internet, intranets and extranets all serve the purpose of sharing data and information.
Internet
The Internet is a worldwide network of computer networks that use the TCP/IP network protocols to facilitate data transmission and exchange
The Internet carries a vast array of information resources and services and the infrastructure to support electronic mail
The internet is not owned by any particular person or organisation
Internet can be accessed by public via an ISP
Intranet
Intranet is a private network maintained by a company or government organization and is based on the Internet Protocol (IP) to securely share any part of an organization's information or Network Operating Systems within that organization
Intranet can only be accessed by specific users
Extranet
Extranets are an extension of an intranet to allow access to customers and other users outside the organisation
Extranet like intranet, but external access also available to specified bodies