An intranet uses internet technology, i.e. a TCP/IP network, to share information within your organisation while being closed to the general public. The intranet may or may not be connected to the internet, but generally is linked to it and protected by a firewall.

An extranet is an intranet extended to include your business partners. It is closed to the general public, but generally connected to the internet and protected from it by SSL (Secure Sockets Layers).