An intranet is a network within an organization. You will find organizations use intranets to have information available for their employees and possibly customers or suppliers that they do not have available for everyone in the world on the Internet.
Items that you may often find on an intranet are training materials for the organization, documents meant only for use by employees such as pricing and other proprietary documents. Employee manuals would be on an intranet. Within a college you may find detail information on an intranet and only available to current students. Student grades and financial aid information will quite often be materials for an intranet.
The word intranet is similar to two other words you see often so lets define them also.
The Internet that we hear about all the time and almost everyone uses is actually a network of networks that are connected using common protocols and connected to everywhere people access this large network. It includes networks in government agencies, colleges, schools, personal networks and business networks. It should normally start with a capital (uppercase) I as it is a specific internet.
An internet (small i) is a network of several networks. For instance the state of Georgia has an internet consisting of all the networks at the different agencies and colleges and schools. GNTC has an internet consisting of the networks at each campus. An internet may connect to (thereby be part of) the Internet. The Internet is an internet.
internet is a shortened version of internetwork but almost always called internet.