An extranet is a website that allows controlled access to partners, vendors and suppliers or an authorised set of customers - normally to a subset of the information accessible from an organization's intranet. An extranet is similar to a DMZ in that it provides access to needed services for authorised parties, without granting access to an organization's entire network.
Historically the term was occasionally also used in the sense of an two organisation sharing their internal networks over a VPN[1]