An extranet is a private network that uses Internet protocols, network connectivity, and possibly the public telecommunication system to securely share part of a business's information or operations with suppliers, vendors, partners, customers or other businesses. An extranet can be viewed as part of a company's Intranet that is extended to users outside the company (eg: normally over the Internet). It has also been described as a "state of mind" in which the Internet is perceived as a way to do business with other companies as well as to sell products to customers.

Briefly, an extranet can be understood as "a private internet over the Internet".

An argument has been made that "extranet" is just a buzzword for describing what institutions have been doing for decades, that is, interconnecting to each other to create private networks for sharing information.

Another very common use of the term "extranet" is to designate the "private part" of a website, where "registered users" can navigate, enabled by authentication mechanisms on a "login page".