Originally answered Jan 27, 2018
The Internet is the global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to link devices worldwide.
An intranet is a private computer network that uses Internet protocols, network connectivity, and possibly the public telecommunication system to securely share part of an organization’s information or operations with its employees.
An extranet is an intranet that can be partially accessed by authorized outside users, enabling businesses to exchange information over the Internet securely. It is a controlled private network that allows access to partners, vendors and suppliers or an authorized set of customers — normally to a subset of the information accessible from an organization’s intranet.