An Intranet is simply a website only accessible within a business that makes the information the employees need available to them via their web browsers. All that is required is a web server connected to the business LAN on which the information is stored.
An Intranet becomes a really powerful business tool however, when it utilises a database to store the business's data. Storing the data in a database allows the data to be correlated and displayed in various forms far more flexibly than could be achieved with static web pages.
For example: A business could maintain a list of "Terms and Conditions" documents that apply to various classes of client, as well as a list of which clients were of which class. The employee can then look up the client and find what class they are in. They can then look up which "Terms and Conditions" document applies to that class. However if the data was stored in a database instead of in static lists, then retrieving the client details could be made to include a clickable link to the "Terms and Conditions" document appropriate to the client. Also, when a "Terms and Conditions" document is updated, a list of clients affected can be generated.