An intranet is a network to be used inside an organization
It is a network that has things for the people within the organization. Forms that are used by the employees may be on the intranet. Information such as procedures to do work or organizational procedures will be typically on an intranet.
Accounting information, student grades and sales information would be on an intranet. Programs that collect data from employees will be on an intranet along with many areas of postings of information for employees. Some of the postings could be company information, but other things on an intranet may be things like who has a birthday today, worked here for a 5 year anniversary, and other things you may have used to find on a bulletin board in newsletter.
Sometimes parts of the intranet are open to select outsiders that have vested interests in the area.
The intranet is for inside the organization, where as the web page and other type information for outside the organization is posted on the Internet or internet.