internet, intranet, extranet

Use internet to refer to the worldwide collection of networks that use open protocols such as TCP/IP to communicate with one another. Don't capitalize.

Use intranet to refer to a communications network based on web technology but that's available only to certain people, such as the employees of a company. Don't capitalize.

Use extranet to refer to an extension of an intranet that uses internet protocols to give authorized outside users limited access to the intranet. Don't capitalize.