What is the difference between the Internet, Intranet and Extranet?

The Internet is an open means of communicating with any computer using a communications protocol called Transmission Control Protocol/Internet Protocol or TCP/IP.
This usually takes the form of web pages that are accessed through a web browser.

An Intranet uses the same underlying technology as the internet but is a closed system that is on a local network and cannot be seem by people outside of that network. This usually takes the form of an internal company Intranet that uses web pages to disseminate information to a company workforce without the information being accessible to the general public.

An Extranet is usually built for the same purposes as an Intranet i.e. to inform a company workforce, however, this is accessible via the internet. Unauthorised access is prevented via username and password protection. This allows employees to view company details outside of work networks, for example at home.