Intranet is a connection that runs in local area ,like in a small city used to communicate the staffs with Central Administration support

Extranet is a platform used by an organization to handle it's clients like surveying their comments

Internet is the giant network,which is the inter connection of small networks. Internet means internetwork as you see from the name.

Intranet is a network that runs in a small limited area. It's internal. Internet is a world wide network that connects many devises to a central network.