An extranet actually combines both the Internet and an intranet. It extends an intranet, or internal network, to other users over the Internet. Most extranets can be accessed via a Web interface using a Web browser. Since secure or confidential information is often accessible within an intranet, extranets typically require authentication for users to access them.
Extranets are often used by companies that need to share selective information with other businesses or individuals. For example, a supplier may use an extranet to provide inventory data to certain clients, while not making the information available to the general public. The extranet may also include a secure means of communication for the company and its clients, such as a support ticket system or Web-based forum.
Unlike the Internet, "extranet" is not a proper noun and therefore should not be capitalized.