The differences between the Internet, an intranet and an extranet is the size of interconnected computer networks within each of them and the accessibility of the information shared within the networks. The Internet is a global system of interconnected computer networks that share and have access to massive amounts of data, while an intranet is a private network of computers that is confined within an enterprise. The extranet, on the other hand, is somewhat like an intranet that can be accessed externally or can be thought of as an extended intranet.
The Internet connects millions of computer networks, from private, public, academic and business establishments, sharing information, resources and services. People all over the world use the Internet to access web pages, send emails, play online games, listen to audio files, watch videos and download data.
An intranet network of computers is used primarily by companies and other private organizations. It behaves just like the Internet but the information shared, the pages, communication and other intranet features are contained within the intranet's firewall, to prevent unauthorized access from external computer networks.
The extranet is basically an intranet that allows access to external computers or computer networks. The type of access given to external computers in an extranet may have varying levels. Extranets are often used by businesses to give access to its costumers, suppliers, partners and vendors.