What is the intranet?

An intranet is a 'private' version of the internet. It's a network of computers using the same technology as the internet. An intranet is not reachable from the internet (without proper security tools) and doesn't even have to be connected to the internet.
Because an intranet can't be reached from the internet without special security measures it's not as sensitive to cyber attacks as the internet is.
http://en.wikipedia.org/wiki/Intranet