In networking, there are various forms of architecture.  Many are familiar with the internet but may be unaware as to what intranets and extranets are.   Here a quick breakdown of the three:

Internet – Is public and is meant for everyone’s use.

Intranet – Is your internal network.  The network you use at work or school would be considered an intranet.  An intranet is only meant for use for individuals on that network.

Extranet – Is an external network used for outsiders.  For example, you are working with an organization that you don’t quite trust so you set up an extranet.  Giving the other organization access to that external network without giving them access to your internal network.